Check for orders which are cancelled=False instead of those who are paid to avoid "overselling".

src/shop/models.py

@@ -418,7 +418,7 @@ class Product(CreatedUpdatedModel, UUIDModel):
         if self.stock_amount:
             sold = OrderProductRelation.objects.filter(
                 product=self,
-                order__paid=True,
+                order__cancelled=False,
             ).aggregate(Sum('quantity'))['quantity__sum']
 
             total_left = self.stock_amount - (sold or 0)

src/shop/tests.py

@@ -22,16 +22,21 @@ class ProductAvailabilityTest(TestCase):
         """ If max orders have been made, the product is NOT available. """
         product = ProductFactory(stock_amount=2)
 
-        for i in range(2):
-            opr = OrderProductRelationFactory(product=product)
-            order = opr.order
-            order.paid = True
-            order.save()
+        opr1 = OrderProductRelationFactory(product=product)
+        opr2 = OrderProductRelationFactory(product=product)
 
         self.assertEqual(product.left_in_stock, 0)
         self.assertFalse(product.is_stock_available)
         self.assertFalse(product.is_available())
 
+        # Cancel one order
+        opr1.order.mark_as_cancelled()
+
+        self.assertEqual(product.left_in_stock, 1)
+        self.assertTrue(product.is_stock_available)
+        self.assertTrue(product.is_available())
+
+
     def test_product_available_by_time(self):
         """ The product is available if now is in the right timeframe. """
         product = ProductFactory()