fix

core/apps/orders/serializers/order.py

@@ -20,6 +20,7 @@ class OrderItemCreateSerializer(serializers.Serializer):
             raise serializers.ValidationError("Product not found")
         data['product'] = product
         product.quantity_left -= round(data['quantity'] / product.min_quantity)
+        product.save()
         data['price'] = round((data['quantity'] / product.min_quantity) * product.price)
         return data