When an order is posted using the webservice the response is the same as posted, when the order is requested again or looked up in the admin panel the carrier changed to the default carrier. The total order price changes and it gives an payment error + changes the state of the order.
The cart is posted with the same carrier_id.
When the "best price" or "best grade" carrier is selected the carrier is set to carrier_id = 0
Because of this problem it is not possible to post an order where the carrier is not the default carrier.
I put screenshots/code in the attachments as a proof of concept.