diff --git a/registrasion/controllers/invoice.py b/registrasion/controllers/invoice.py
index dafb134b..264049c8 100644
--- a/registrasion/controllers/invoice.py
+++ b/registrasion/controllers/invoice.py
@@ -57,6 +57,7 @@ class InvoiceController(object):
         return value
 
     @classmethod
+    @transaction.atomic
     def _generate(cls, cart):
         ''' Generates an invoice for the given cart. '''
         invoice = rego.Invoice.objects.create(
@@ -65,7 +66,6 @@ class InvoiceController(object):
             cart_revision=cart.revision,
             value=Decimal()
         )
-        invoice.save()
 
         product_items = rego.ProductItem.objects.filter(cart=cart)
 
@@ -85,7 +85,6 @@ class InvoiceController(object):
                 quantity=item.quantity,
                 price=product.price,
             )
-            line_item.save()
             invoice_value += line_item.quantity * line_item.price
 
         for item in discount_items:
@@ -95,11 +94,13 @@ class InvoiceController(object):
                 quantity=item.quantity,
                 price=cls.resolve_discount_value(item) * -1,
             )
-            line_item.save()
             invoice_value += line_item.quantity * line_item.price
 
-        # TODO: calculate line items from discounts
         invoice.value = invoice_value
+
+        if invoice.value == 0:
+            invoice.paid = True
+
         invoice.save()
 
         return invoice
diff --git a/registrasion/tests/test_discount.py b/registrasion/tests/test_discount.py
index 1fb4225d..860a90a3 100644
--- a/registrasion/tests/test_discount.py
+++ b/registrasion/tests/test_discount.py
@@ -346,9 +346,8 @@ class DiscountTestCase(RegistrationCartTestCase):
 
         discounts = discount.available_discounts(self.USER_1, [self.CAT_2], [])
         self.assertEqual(2, discounts[0].quantity)
-        inv = InvoiceController.for_cart(cart.cart)
-        inv.pay("Dummy reference", inv.invoice.value)
-        self.assertTrue(inv.invoice.paid)
+        cart.cart.active = False
+        cart.cart.save()
 
     def test_discount_quantity_is_correct_after_first_purchase(self):
         self.test_discount_quantity_is_correct_before_first_purchase()
@@ -358,9 +357,8 @@ class DiscountTestCase(RegistrationCartTestCase):
 
         discounts = discount.available_discounts(self.USER_1, [self.CAT_2], [])
         self.assertEqual(1, discounts[0].quantity)
-        inv = InvoiceController.for_cart(cart.cart)
-        inv.pay("Dummy reference", inv.invoice.value)
-        self.assertTrue(inv.invoice.paid)
+        cart.cart.active = False
+        cart.cart.save()
 
     def test_discount_is_gone_after_quantity_exhausted(self):
         self.test_discount_quantity_is_correct_after_first_purchase()
diff --git a/registrasion/tests/test_invoice.py b/registrasion/tests/test_invoice.py
index fd6c9cfb..5dc1559b 100644
--- a/registrasion/tests/test_invoice.py
+++ b/registrasion/tests/test_invoice.py
@@ -83,18 +83,16 @@ class InvoiceTestCase(RegistrationCartTestCase):
             code="VOUCHER",
             limit=1
         )
-        voucher.save()
         discount = rego.VoucherDiscount.objects.create(
             description="VOUCHER RECIPIENT",
             voucher=voucher,
         )
-        discount.save()
         rego.DiscountForProduct.objects.create(
             discount=discount,
             product=self.PROD_1,
             percentage=Decimal(50),
             quantity=1
-        ).save()
+        )
 
         current_cart = TestingCartController.for_user(self.USER_1)
         current_cart.apply_voucher(voucher.code)
@@ -111,6 +109,32 @@ class InvoiceTestCase(RegistrationCartTestCase):
             self.PROD_1.price * Decimal("0.5"),
             invoice_1.invoice.value)
 
+    def test_zero_value_invoice_is_automatically_paid(self):
+        voucher = rego.Voucher.objects.create(
+            recipient="Voucher recipient",
+            code="VOUCHER",
+            limit=1
+        )
+        discount = rego.VoucherDiscount.objects.create(
+            description="VOUCHER RECIPIENT",
+            voucher=voucher,
+        )
+        rego.DiscountForProduct.objects.create(
+            discount=discount,
+            product=self.PROD_1,
+            percentage=Decimal(100),
+            quantity=1
+        )
+
+        current_cart = TestingCartController.for_user(self.USER_1)
+        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)
+        invoice_1 = InvoiceController.for_cart(current_cart.cart)
+
+        self.assertTrue(invoice_1.invoice.paid)
+
     def test_invoice_voids_self_if_cart_is_invalid(self):
         current_cart = TestingCartController.for_user(self.USER_1)
 
@@ -169,3 +193,8 @@ class InvoiceTestCase(RegistrationCartTestCase):
 
         with self.assertRaises(ValidationError):
             invoice_1.void()
+
+    def test_cannot_generate_blank_invoice(self):
+        current_cart = TestingCartController.for_user(self.USER_1)
+        with self.assertRaises(ValidationError):
+            invoice_1 = InvoiceController.for_cart(current_cart.cart)
diff --git a/registrasion/tests/test_voucher.py b/registrasion/tests/test_voucher.py
index e3ac0d0d..1502c2fc 100644
--- a/registrasion/tests/test_voucher.py
+++ b/registrasion/tests/test_voucher.py
@@ -125,8 +125,8 @@ class VoucherTestCases(RegistrationCartTestCase):
         current_cart = TestingCartController.for_user(self.USER_1)
         current_cart.apply_voucher(voucher.code)
 
-        inv = InvoiceController.for_cart(current_cart.cart)
-        inv.pay("Hello!", inv.invoice.value)
+        current_cart.cart.active = False
+        current_cart.cart.save()
 
         current_cart = TestingCartController.for_user(self.USER_1)
 
@@ -139,9 +139,11 @@ class VoucherTestCases(RegistrationCartTestCase):
         voucher = self.new_voucher(limit=2)
         current_cart = TestingCartController.for_user(self.USER_1)
         current_cart.apply_voucher(voucher.code)
+        current_cart.add_to_cart(self.PROD_1, 1)
 
         inv = InvoiceController.for_cart(current_cart.cart)
-        inv.pay("Hello!", inv.invoice.value)
+        if not inv.invoice.paid:
+            inv.pay("Hello!", inv.invoice.value)
 
         current_cart = TestingCartController.for_user(self.USER_1)
         with self.assertRaises(ValidationError):