diff --git a/registrasion/admin.py b/registrasion/admin.py
index e00b58ea..17969cd1 100644
--- a/registrasion/admin.py
+++ b/registrasion/admin.py
@@ -80,3 +80,15 @@ class VoucherAdmin(nested_admin.NestedAdmin):
         VoucherDiscountInline,
         VoucherEnablingConditionInline,
     ]
+
+
+# Enabling conditions
+@admin.register(rego.ProductEnablingCondition)
+class ProductEnablingConditionAdmin(nested_admin.NestedAdmin):
+    model = rego.ProductEnablingCondition
+
+
+# Enabling conditions
+@admin.register(rego.CategoryEnablingCondition)
+class CategoryEnablingConditionAdmin(nested_admin.NestedAdmin):
+    model = rego.CategoryEnablingCondition
diff --git a/registrasion/controllers/cart.py b/registrasion/controllers/cart.py
index e9532bf5..59b308ed 100644
--- a/registrasion/controllers/cart.py
+++ b/registrasion/controllers/cart.py
@@ -54,57 +54,91 @@ class CartController(object):
         self.cart.time_last_updated = timezone.now()
         self.cart.reservation_duration = max(reservations)
 
-    def add_to_cart(self, product, quantity):
-        ''' Adds _quantity_ of the given _product_ to the cart. Raises
-        ValidationError if constraints are violated.'''
-
-        prod = ProductController(product)
-
-        # TODO: Check enabling conditions for product for user
-
-        if not prod.can_add_with_enabling_conditions(self.cart.user, quantity):
-            raise ValidationError("Not enough of that product left (ec)")
-
-        if not prod.user_can_add_within_limit(self.cart.user, quantity):
-            raise ValidationError("Not enough of that product left (user)")
-
-        try:
-            # Try to update an existing item within this cart if possible.
-            product_item = rego.ProductItem.objects.get(
-                cart=self.cart,
-                product=product)
-            product_item.quantity += quantity
-        except ObjectDoesNotExist:
-            product_item = rego.ProductItem.objects.create(
-                cart=self.cart,
-                product=product,
-                quantity=quantity,
-            )
-        product_item.save()
-
+    def end_batch(self):
+        ''' Performs operations that occur occur at the end of a batch of
+        product changes/voucher applications etc. '''
         self.recalculate_discounts()
 
         self.extend_reservation()
         self.cart.revision += 1
         self.cart.save()
 
-    def apply_voucher(self, voucher):
-        ''' Applies the given voucher to this cart. '''
+    def set_quantity(self, product, quantity, batched=False):
+        ''' Sets the _quantity_ of the given _product_ in the cart to the given
+        _quantity_. '''
+
+        if quantity < 0:
+            raise ValidationError("Cannot have fewer than 0 items in cart.")
+
+        try:
+            product_item = rego.ProductItem.objects.get(
+                cart=self.cart,
+                product=product)
+            old_quantity = product_item.quantity
+
+            if quantity == 0:
+                product_item.delete()
+                return
+        except ObjectDoesNotExist:
+            if quantity == 0:
+                return
+
+            product_item = rego.ProductItem.objects.create(
+                cart=self.cart,
+                product=product,
+                quantity=0,
+            )
+
+            old_quantity = 0
+
+        # Validate the addition to the cart
+        adjustment = quantity - old_quantity
+        prod = ProductController(product)
+
+        if not prod.can_add_with_enabling_conditions(
+                self.cart.user, adjustment):
+            raise ValidationError("Not enough of that product left (ec)")
+
+        if not prod.user_can_add_within_limit(self.cart.user, adjustment):
+            raise ValidationError("Not enough of that product left (user)")
+
+        product_item.quantity = quantity
+        product_item.save()
+
+        if not batched:
+            self.end_batch()
+
+    def add_to_cart(self, product, quantity):
+        ''' Adds _quantity_ of the given _product_ to the cart. Raises
+        ValidationError if constraints are violated.'''
+
+        try:
+            product_item = rego.ProductItem.objects.get(
+                cart=self.cart,
+                product=product)
+            old_quantity = product_item.quantity
+        except ObjectDoesNotExist:
+            old_quantity = 0
+        self.set_quantity(product, old_quantity + quantity)
+
+    def apply_voucher(self, voucher_code):
+        ''' Applies the voucher with the given code to this cart. '''
 
         # TODO: is it valid for a cart to re-add a voucher that they have?
 
         # Is voucher exhausted?
         active_carts = rego.Cart.reserved_carts()
+
+        # Try and find the voucher
+        voucher = rego.Voucher.objects.get(code=voucher_code.upper())
+
         carts_with_voucher = active_carts.filter(vouchers=voucher)
         if len(carts_with_voucher) >= voucher.limit:
             raise ValidationError("This voucher is no longer available")
 
         # If successful...
         self.cart.vouchers.add(voucher)
-
-        self.extend_reservation()
-        self.cart.revision += 1
-        self.cart.save()
+        self.end_batch()
 
     def validate_cart(self):
         ''' Determines whether the status of the current cart is valid;
@@ -153,7 +187,12 @@ class CartController(object):
         # Delete the existing entries.
         rego.DiscountItem.objects.filter(cart=self.cart).delete()
 
-        for item in self.cart.productitem_set.all():
+        # The highest-value discounts will apply to the highest-value
+        # products first.
+        product_items = self.cart.productitem_set.all()
+        product_items = product_items.order_by('product__price')
+        product_items = reversed(product_items)
+        for item in product_items:
             self._add_discount(item.product, item.quantity)
 
     def _add_discount(self, product, quantity):
@@ -172,9 +211,7 @@ class CartController(object):
             # Get the count of past uses of this discount condition
             # as this affects the total amount we're allowed to use now.
             past_uses = rego.DiscountItem.objects.filter(
-                cart__active=False,
                 discount=discount.discount,
-                product=product,
             )
             agg = past_uses.aggregate(Sum("quantity"))
             past_uses = agg["quantity__sum"]
diff --git a/registrasion/controllers/invoice.py b/registrasion/controllers/invoice.py
index 5ce562f3..e9a654e4 100644
--- a/registrasion/controllers/invoice.py
+++ b/registrasion/controllers/invoice.py
@@ -59,20 +59,23 @@ class InvoiceController(object):
 
         # TODO: calculate line items.
         product_items = rego.ProductItem.objects.filter(cart=cart)
+        product_items = product_items.order_by(
+            "product__category__order", "product__order"
+        )
         discount_items = rego.DiscountItem.objects.filter(cart=cart)
         invoice_value = Decimal()
         for item in product_items:
+            product = item.product
             line_item = rego.LineItem.objects.create(
                 invoice=invoice,
-                description=item.product.name,
+                description="%s - %s" % (product.category.name, product.name),
                 quantity=item.quantity,
-                price=item.product.price,
+                price=product.price,
             )
             line_item.save()
             invoice_value += line_item.quantity * line_item.price
 
         for item in discount_items:
-
             line_item = rego.LineItem.objects.create(
                 invoice=invoice,
                 description=item.discount.description,
diff --git a/registrasion/forms.py b/registrasion/forms.py
new file mode 100644
index 00000000..54b3115a
--- /dev/null
+++ b/registrasion/forms.py
@@ -0,0 +1,55 @@
+import models as rego
+
+from django import forms
+
+
+def CategoryForm(category):
+
+    PREFIX = "product_"
+
+    def field_name(product):
+        return PREFIX + ("%d" % product.id)
+
+    class _CategoryForm(forms.Form):
+
+        @staticmethod
+        def initial_data(product_quantities):
+            ''' Prepares initial data for an instance of this form.
+            product_quantities is a sequence of (product,quantity) tuples '''
+            initial = {}
+            for product, quantity in product_quantities:
+                initial[field_name(product)] = quantity
+
+            return initial
+
+        def product_quantities(self):
+            ''' Yields a sequence of (product, quantity) tuples from the
+            cleaned form data. '''
+            for name, value in self.cleaned_data.items():
+                if name.startswith(PREFIX):
+                    product_id = int(name[len(PREFIX):])
+                    yield (product_id, value, name)
+
+        def disable_product(self, product):
+            ''' Removes a given product from this form. '''
+            del self.fields[field_name(product)]
+
+    products = rego.Product.objects.filter(category=category).order_by("order")
+    for product in products:
+
+        help_text = "$%d -- %s" % (product.price, product.description)
+
+        field = forms.IntegerField(
+            label=product.name,
+            help_text=help_text,
+        )
+        _CategoryForm.base_fields[field_name(product)] = field
+
+    return _CategoryForm
+
+class VoucherForm(forms.Form):
+    voucher = forms.CharField(
+        label="Voucher code",
+        help_text="If you have a voucher code, enter it here",
+        required=True,
+    )
diff --git a/registrasion/migrations/0001_initial.py b/registrasion/migrations/0001_squashed_0002_auto_20160304_1723.py
similarity index 99%
rename from registrasion/migrations/0001_initial.py
rename to registrasion/migrations/0001_squashed_0002_auto_20160304_1723.py
index ab78d1f0..cc1e3f0d 100644
--- a/registrasion/migrations/0001_initial.py
+++ b/registrasion/migrations/0001_squashed_0002_auto_20160304_1723.py
@@ -9,6 +9,8 @@ from django.conf import settings
 
 class Migration(migrations.Migration):
 
+    replaces = [('registrasion', '0001_initial'), ('registrasion', '0002_auto_20160304_1723')]
+
     dependencies = [
         migrations.swappable_dependency(settings.AUTH_USER_MODEL),
     ]
@@ -225,12 +227,12 @@ class Migration(migrations.Migration):
         migrations.AddField(
             model_name='enablingconditionbase',
             name='categories',
-            field=models.ManyToManyField(to=b'registrasion.Category'),
+            field=models.ManyToManyField(to=b'registrasion.Category', blank=True),
         ),
         migrations.AddField(
             model_name='enablingconditionbase',
             name='products',
-            field=models.ManyToManyField(to=b'registrasion.Product'),
+            field=models.ManyToManyField(to=b'registrasion.Product', blank=True),
         ),
         migrations.AddField(
             model_name='discountitem',
diff --git a/registrasion/models.py b/registrasion/models.py
index 4866c00b..96ecf219 100644
--- a/registrasion/models.py
+++ b/registrasion/models.py
@@ -99,6 +99,11 @@ class Voucher(models.Model):
     def __str__(self):
         return "Voucher for %s" % self.recipient
 
+    def save(self, *a, **k):
+        ''' Normalise the voucher code to be uppercase '''
+        self.code = self.code.upper()
+        super(Voucher, self).save(*a, **k)
+
     recipient = models.CharField(max_length=64, verbose_name=_("Recipient"))
     code = models.CharField(max_length=16,
                             unique=True,
@@ -149,13 +154,13 @@ class DiscountForProduct(models.Model):
         cats = DiscountForCategory.objects.filter(
             discount=self.discount,
             category=self.product.category)
-        if len(prods) > 1 or self not in prods:
+        if len(prods) > 1:
             raise ValidationError(
                 _("You may only have one discount line per product"))
         if len(cats) != 0:
             raise ValidationError(
                 _("You may only have one discount for "
-                "a product or its category"))
+                    "a product or its category"))
 
     discount = models.ForeignKey(DiscountBase, on_delete=models.CASCADE)
     product = models.ForeignKey(Product, on_delete=models.CASCADE)
@@ -184,8 +189,8 @@ class DiscountForCategory(models.Model):
         if len(prods) != 0:
             raise ValidationError(
                 _("You may only have one discount for "
-                "a product or its category"))
-        if len(cats) > 1 or self not in cats:
+                    "a product or its category"))
+        if len(cats) > 1:
             raise ValidationError(
                 _("You may only have one discount line per category"))
 
@@ -257,8 +262,8 @@ class EnablingConditionBase(models.Model):
 
     description = models.CharField(max_length=255)
     mandatory = models.BooleanField(default=False)
-    products = models.ManyToManyField(Product)
-    categories = models.ManyToManyField(Category)
+    products = models.ManyToManyField(Product, blank=True)
+    categories = models.ManyToManyField(Category, blank=True)
 
 
 class TimeOrStockLimitEnablingCondition(EnablingConditionBase):
diff --git a/registrasion/templates/invoice.html b/registrasion/templates/invoice.html
new file mode 100644
index 00000000..a66132cf
--- /dev/null
+++ b/registrasion/templates/invoice.html
@@ -0,0 +1,37 @@
+<!--- Sample template. Move elsewhere once it's ready to go. -->
+
+{% extends "site_base.html" %}
+{% block body %}
+
+<h1>Invoice {{ invoice.id }}</h1>
+
+<ul>
+  <li>Void: {{ invoice.void }}</li>
+  <li>Paid: {{ invoice.paid }}</li>
+</ul>
+
+<table>
+  <tr>
+    <th>Description</th>
+    <th>Quantity</th>
+    <th>Price/Unit</th>
+    <th>Total</th>
+  </tr>
+  {% for line_item in invoice.lineitem_set.all %}
+    <tr>
+      <td>{{line_item.description}}</td>
+      <td>{{line_item.quantity}}</td>
+      <td>{{line_item.price}}</td>
+      <td><!-- multiply --> FIXME</td>
+    </tr>
+  {% endfor %}
+  <tr>
+    <th>TOTAL</th>
+    <td></td>
+    <td></td>
+    <td>{{ invoice.value }}</td>
+  </tr>
+</table>
+
+
+{% endblock %}
diff --git a/registrasion/templates/product_category.html b/registrasion/templates/product_category.html
new file mode 100644
index 00000000..0e567bf0
--- /dev/null
+++ b/registrasion/templates/product_category.html
@@ -0,0 +1,28 @@
+<!--- Sample template. Move elsewhere once it's ready to go. -->
+
+{% extends "site_base.html" %}
+{% block body %}
+
+  <h1>Product Category: {{ category.name }}</h1>
+
+  <p>{{ category.description }}</p>
+
+  <form method="post" action="">
+    {% csrf_token %}
+
+    <table>
+        {{ voucher_form }}
+    </table>
+
+    <input type="submit">
+
+    <table>
+        {{ form }}
+    </table>
+
+    <input type="submit">
+
+  </form>
+
+
+{% endblock %}
diff --git a/registrasion/tests/test_cart.py b/registrasion/tests/test_cart.py
index c0c0bc49..c65011b9 100644
--- a/registrasion/tests/test_cart.py
+++ b/registrasion/tests/test_cart.py
@@ -3,6 +3,7 @@ import pytz
 
 from decimal import Decimal
 from django.contrib.auth.models import User
+from django.core.exceptions import ObjectDoesNotExist
 from django.core.exceptions import ValidationError
 from django.test import TestCase
 
@@ -81,7 +82,18 @@ class RegistrationCartTestCase(SetTimeMixin, TestCase):
             limit_per_user=10,
             order=10,
         )
-        cls.PROD_2.save()
+        cls.PROD_3.save()
+
+        cls.PROD_4 = rego.Product.objects.create(
+            name="Product 4",
+            description="This is a test product. It costs $5. "
+                        "A user may have 10 of them.",
+            category=cls.CAT_2,
+            price=Decimal("5.00"),
+            limit_per_user=10,
+            order=10,
+        )
+        cls.PROD_4.save()
 
     @classmethod
     def make_ceiling(cls, name, limit=None, start_time=None, end_time=None):
@@ -159,6 +171,38 @@ class BasicCartTests(RegistrationCartTestCase):
         item = items[0]
         self.assertEquals(2, item.quantity)
 
+    def test_set_quantity(self):
+        current_cart = CartController.for_user(self.USER_1)
+
+        def get_item():
+            return rego.ProductItem.objects.get(
+                cart=current_cart.cart,
+                product=self.PROD_1)
+
+        current_cart.set_quantity(self.PROD_1, 1)
+        self.assertEqual(1, get_item().quantity)
+
+        # Setting the quantity to zero should remove the entry from the cart.
+        current_cart.set_quantity(self.PROD_1, 0)
+        with self.assertRaises(ObjectDoesNotExist):
+            get_item()
+
+        current_cart.set_quantity(self.PROD_1, 9)
+        self.assertEqual(9, get_item().quantity)
+
+        with self.assertRaises(ValidationError):
+            current_cart.set_quantity(self.PROD_1, 11)
+
+        self.assertEqual(9, get_item().quantity)
+
+        with self.assertRaises(ValidationError):
+            current_cart.set_quantity(self.PROD_1, -1)
+
+        self.assertEqual(9, get_item().quantity)
+
+        current_cart.set_quantity(self.PROD_1, 2)
+        self.assertEqual(2, get_item().quantity)
+
     def test_add_to_cart_per_user_limit(self):
         current_cart = CartController.for_user(self.USER_1)
 
diff --git a/registrasion/tests/test_discount.py b/registrasion/tests/test_discount.py
index c0709e8e..5d325eff 100644
--- a/registrasion/tests/test_discount.py
+++ b/registrasion/tests/test_discount.py
@@ -29,7 +29,10 @@ class DiscountTestCase(RegistrationCartTestCase):
         return discount
 
     @classmethod
-    def add_discount_prod_1_includes_cat_2(cls, amount=Decimal(100)):
+    def add_discount_prod_1_includes_cat_2(
+            cls,
+            amount=Decimal(100),
+            quantity=2):
         discount = rego.IncludedProductDiscount.objects.create(
             description="PROD_1 includes CAT_2 " + str(amount) + "%",
         )
@@ -40,7 +43,7 @@ class DiscountTestCase(RegistrationCartTestCase):
             discount=discount,
             category=cls.CAT_2,
             percentage=amount,
-            quantity=2
+            quantity=quantity,
         ).save()
         return discount
 
@@ -169,3 +172,31 @@ class DiscountTestCase(RegistrationCartTestCase):
 
         discount_items = list(cart.cart.discountitem_set.all())
         self.assertEqual(2, discount_items[0].quantity)
+
+    def test_category_discount_applies_once_per_category(self):
+        self.add_discount_prod_1_includes_cat_2(quantity=1)
+        cart = CartController.for_user(self.USER_1)
+        cart.add_to_cart(self.PROD_1, 1)
+
+        # Add two items from category 2
+        cart.add_to_cart(self.PROD_3, 1)
+        cart.add_to_cart(self.PROD_4, 1)
+
+        discount_items = list(cart.cart.discountitem_set.all())
+        # There is one discount, and it should apply to one item.
+        self.assertEqual(1, len(discount_items))
+        self.assertEqual(1, discount_items[0].quantity)
+
+    def test_category_discount_applies_to_highest_value(self):
+        self.add_discount_prod_1_includes_cat_2(quantity=1)
+        cart = CartController.for_user(self.USER_1)
+        cart.add_to_cart(self.PROD_1, 1)
+
+        # Add two items from category 2, add the less expensive one first
+        cart.add_to_cart(self.PROD_4, 1)
+        cart.add_to_cart(self.PROD_3, 1)
+
+        discount_items = list(cart.cart.discountitem_set.all())
+        # There is one discount, and it should apply to the more expensive.
+        self.assertEqual(1, len(discount_items))
+        self.assertEqual(self.PROD_3, discount_items[0].product)
diff --git a/registrasion/tests/test_invoice.py b/registrasion/tests/test_invoice.py
index 4d0360ce..637e82db 100644
--- a/registrasion/tests/test_invoice.py
+++ b/registrasion/tests/test_invoice.py
@@ -91,7 +91,7 @@ class InvoiceTestCase(RegistrationCartTestCase):
         ).save()
 
         current_cart = CartController.for_user(self.USER_1)
-        current_cart.apply_voucher(voucher)
+        current_cart.apply_voucher(voucher.code)
 
         # Should be able to create an invoice after the product is added
         current_cart.add_to_cart(self.PROD_1, 1)
diff --git a/registrasion/tests/test_voucher.py b/registrasion/tests/test_voucher.py
index 169a1b79..50b83417 100644
--- a/registrasion/tests/test_voucher.py
+++ b/registrasion/tests/test_voucher.py
@@ -3,6 +3,7 @@ import pytz
 
 from decimal import Decimal
 from django.core.exceptions import ValidationError
+from django.db import IntegrityError
 
 from registrasion import models as rego
 from registrasion.controllers.cart import CartController
@@ -15,10 +16,10 @@ UTC = pytz.timezone('UTC')
 class VoucherTestCases(RegistrationCartTestCase):
 
     @classmethod
-    def new_voucher(self):
+    def new_voucher(self, code="VOUCHER"):
         voucher = rego.Voucher.objects.create(
             recipient="Voucher recipient",
-            code="VOUCHER",
+            code=code,
             limit=1
         )
         voucher.save()
@@ -30,18 +31,18 @@ class VoucherTestCases(RegistrationCartTestCase):
         self.set_time(datetime.datetime(2015, 01, 01, tzinfo=UTC))
 
         cart_1 = CartController.for_user(self.USER_1)
-        cart_1.apply_voucher(voucher)
+        cart_1.apply_voucher(voucher.code)
         self.assertIn(voucher, cart_1.cart.vouchers.all())
 
         # Second user should not be able to apply this voucher (it's exhausted)
         cart_2 = CartController.for_user(self.USER_2)
         with self.assertRaises(ValidationError):
-            cart_2.apply_voucher(voucher)
+            cart_2.apply_voucher(voucher.code)
 
         # After the reservation duration
         # user 2 should be able to apply voucher
         self.add_timedelta(rego.Voucher.RESERVATION_DURATION * 2)
-        cart_2.apply_voucher(voucher)
+        cart_2.apply_voucher(voucher.code)
         cart_2.cart.active = False
         cart_2.cart.save()
 
@@ -49,7 +50,7 @@ class VoucherTestCases(RegistrationCartTestCase):
         # voucher, as user 2 has paid for their cart.
         self.add_timedelta(rego.Voucher.RESERVATION_DURATION * 2)
         with self.assertRaises(ValidationError):
-            cart_1.apply_voucher(voucher)
+            cart_1.apply_voucher(voucher.code)
 
     def test_voucher_enables_item(self):
         voucher = self.new_voucher()
@@ -69,7 +70,7 @@ class VoucherTestCases(RegistrationCartTestCase):
             current_cart.add_to_cart(self.PROD_1, 1)
 
         # Apply the voucher
-        current_cart.apply_voucher(voucher)
+        current_cart.apply_voucher(voucher.code)
         current_cart.add_to_cart(self.PROD_1, 1)
 
     def test_voucher_enables_discount(self):
@@ -89,6 +90,20 @@ class VoucherTestCases(RegistrationCartTestCase):
 
         # Having PROD_1 in place should add a discount
         current_cart = CartController.for_user(self.USER_1)
-        current_cart.apply_voucher(voucher)
+        current_cart.apply_voucher(voucher.code)
         current_cart.add_to_cart(self.PROD_1, 1)
         self.assertEqual(1, len(current_cart.cart.discountitem_set.all()))
+
+    def test_voucher_codes_unique(self):
+        voucher1 = self.new_voucher(code="VOUCHER")
+        with self.assertRaises(IntegrityError):
+            voucher2 = self.new_voucher(code="VOUCHER")
+
+    def test_multiple_vouchers_work(self):
+        voucher1 = self.new_voucher(code="VOUCHER1")
+        voucher2 = self.new_voucher(code="VOUCHER2")
+
+    def test_vouchers_case_insensitive(self):
+        voucher = self.new_voucher(code="VOUCHeR")
+        current_cart = CartController.for_user(self.USER_1)
+        current_cart.apply_voucher(voucher.code.lower())
diff --git a/registrasion/urls.py b/registrasion/urls.py
new file mode 100644
index 00000000..746d163d
--- /dev/null
+++ b/registrasion/urls.py
@@ -0,0 +1,9 @@
+from django.conf.urls import url, patterns
+
+urlpatterns = patterns(
+    "registrasion.views",
+    url(r"^category/([0-9]+)$", "product_category", name="product_category"),
+    url(r"^checkout$", "checkout", name="checkout"),
+    url(r"^invoice/([0-9]+)$", "invoice", name="invoice"),
+    url(r"^invoice/([0-9]+)/pay$", "pay_invoice", name="pay_invoice"),
+)
diff --git a/registrasion/views.py b/registrasion/views.py
new file mode 100644
index 00000000..3957da7d
--- /dev/null
+++ b/registrasion/views.py
@@ -0,0 +1,135 @@
+from registrasion import forms
+from registrasion import models as rego
+from registrasion.controllers.cart import CartController
+from registrasion.controllers.invoice import InvoiceController
+from registrasion.controllers.product import ProductController
+
+from django.contrib.auth.decorators import login_required
+from django.core.exceptions import ObjectDoesNotExist
+from django.core.exceptions import ValidationError
+from django.db import transaction
+from django.shortcuts import redirect
+from django.shortcuts import render
+
+
+@login_required
+def product_category(request, category_id):
+    ''' Registration selections form for a specific category of items '''
+
+    PRODUCTS_FORM_PREFIX = "products"
+    VOUCHERS_FORM_PREFIX = "vouchers"
+
+    category_id = int(category_id)  # Routing is [0-9]+
+    category = rego.Category.objects.get(pk=category_id)
+    current_cart = CartController.for_user(request.user)
+
+    CategoryForm = forms.CategoryForm(category)
+
+    products = rego.Product.objects.filter(category=category)
+    products = products.order_by("order")
+
+    if request.method == "POST":
+        cat_form = CategoryForm(request.POST, request.FILES, prefix=PRODUCTS_FORM_PREFIX)
+        voucher_form = forms.VoucherForm(request.POST, prefix=VOUCHERS_FORM_PREFIX)
+
+        if voucher_form.is_valid():
+            # Apply voucher
+            # leave
+            voucher = voucher_form.cleaned_data["voucher"]
+            try:
+                current_cart.apply_voucher(voucher)
+            except Exception as e:
+                voucher_form.add_error("voucher", e)
+        elif cat_form.is_valid():
+            try:
+                with transaction.atomic():
+                    for product_id, quantity, field_name \
+                            in cat_form.product_quantities():
+                        product = rego.Product.objects.get(pk=product_id)
+                        try:
+                            current_cart.set_quantity(
+                                product, quantity, batched=True)
+                        except ValidationError as ve:
+                            cat_form.add_error(field_name, ve)
+                    if cat_form.errors:
+                        raise ValidationError("Cannot add that stuff")
+                    current_cart.end_batch()
+            except ValidationError as ve:
+                pass
+
+    else:
+        # Create initial data for each of products in category
+        items = rego.ProductItem.objects.filter(
+            product__category=category,
+            cart=current_cart.cart,
+        )
+        quantities = []
+        for product in products:
+            # Only add items that are enabled.
+            prod = ProductController(product)
+            try:
+                quantity = items.get(product=product).quantity
+            except ObjectDoesNotExist:
+                quantity = 0
+            quantities.append((product, quantity))
+
+        initial = CategoryForm.initial_data(quantities)
+        cat_form = CategoryForm(prefix=PRODUCTS_FORM_PREFIX, initial=initial)
+
+        voucher_form = forms.VoucherForm(prefix=VOUCHERS_FORM_PREFIX)
+
+    for product in products:
+        # Remove fields that do not have an enabling condition.
+        prod = ProductController(product)
+        if not prod.can_add_with_enabling_conditions(request.user, 0):
+            cat_form.disable_product(product)
+
+
+    data = {
+        "category": category,
+        "form": cat_form,
+        "voucher_form": voucher_form,
+    }
+
+    return render(request, "product_category.html", data)
+
+
+@login_required
+def checkout(request):
+    ''' Runs checkout for the current cart of items, ideally generating an
+    invoice. '''
+
+    current_cart = CartController.for_user(request.user)
+    current_invoice = InvoiceController.for_cart(current_cart.cart)
+
+    return redirect("invoice", current_invoice.invoice.id)
+
+
+@login_required
+def invoice(request, invoice_id):
+    ''' Displays an invoice for a given invoice id. '''
+
+    invoice_id = int(invoice_id)
+    inv = rego.Invoice.objects.get(pk=invoice_id)
+    current_invoice = InvoiceController(inv)
+
+    data = {
+        "invoice": current_invoice.invoice,
+    }
+
+    return render(request, "invoice.html", data)
+
+@login_required
+def pay_invoice(request, invoice_id):
+    ''' Marks the invoice with the given invoice id as paid.
+    WORK IN PROGRESS FUNCTION. Must be replaced with real payment workflow.
+
+    '''
+
+    invoice_id = int(invoice_id)
+    inv = rego.Invoice.objects.get(pk=invoice_id)
+    current_invoice = InvoiceController(inv)
+    if not inv.paid and not current_invoice.is_valid():
+        current_invoice.pay("Demo invoice payment", inv.value)
+
+    return redirect("invoice", current_invoice.invoice.id)