Credit note is automatically applied if you have a single invoice

registrasion/controllers/invoice.py

@@ -198,10 +198,30 @@ class InvoiceController(ForId, object):
 
         commerce.LineItem.objects.bulk_create(line_items)
 
+        cls._apply_credit_notes(invoice)
         cls.email_on_invoice_creation(invoice)
 
         return invoice
 
+    @classmethod
+    def _apply_credit_notes(cls, invoice):
+        ''' Applies the user's credit notes to the given invoice on creation.
+        '''
+
+        notes = commerce.CreditNote.objects.filter(invoice__user=invoice.user)
+
+        if len(notes) == 0:
+            return
+
+        for note in notes:
+            try:
+                CreditNoteController(note).apply_to_invoice(invoice)
+            except ValidationError:
+                # ValidationError will get raised once we're overpaying.
+                break
+
+        invoice.refresh_from_db()
+
     def can_view(self, user=None, access_code=None):
         ''' Returns true if the accessing user is allowed to view this invoice,
         or if the given access code matches this invoice's user's access code.