B1ServiceLayer - InvoicesService_Preview to for example calculating end amount of invoice

We are actually working on a invoice import from shop backend. As interface we are using the B1ServiceLayer.

In the B1ServiceLayer are two methods available: OrdersService_Preview and InvoicesService_Preview.

OrdersService_Preview -> Works fine.
InvoicesService_Preview -> "Internal error (-1002) occurred"

Is the InvoicesService_Preview not implemented already?
Is there a difference to the OrdersService_Preview?