Great work! While I was testing the demo site; demo.online-ex.com, I added 3 items from 3 different stores. During checkout, I got the error below;
Error: Shipping total couldn't be calculated. Full exception: Nop.Core.NopException: Shipping total couldn't be calculated at Nop.Services.Orders.OrderProcessingService.PlaceOrder(StoreOrder storeOrder) in c:\Web Sites\Online Store\nopCommerce_2.30_Source\Libraries\Nop.Services\Orders\OrderProcessingService.cs:line 657 Error: Shipping total couldn't be calculated. Full exception: Nop.Core.NopException: Shipping total couldn't be calculated at Nop.Services.Orders.OrderProcessingService.PlaceOrder(StoreOrder storeOrder) in c:\Web Sites\Online Store\nopCommerce_2.30_Source\Libraries\Nop.Services\Orders\OrderProcessingService.cs:line 657
and noticed that the first item was processed. Then I went through the checkout process again and the second item was processed with the error;
Error: Shipping total couldn't be calculated. Full exception: Nop.Core.NopException: Shipping total couldn't be calculated at Nop.Services.Orders.OrderProcessingService.PlaceOrder(StoreOrder storeOrder) in c:\Web Sites\Online Store\nopCommerce_2.30_Source\Libraries\Nop.Services\Orders\OrderProcessingService.cs:line 657
So i am guessing there is a shipping total mix up during checkout.