I am using Amazon's MWS Feeds API to submit carton data using the SubmitFeed call for _POST_FBA_INBOUND_CARTON_CONTENTS_. There are no errors or warnings and all subsequent calls in my InboundShipment to Amazon work just fine. I can even get the labels using the GetUniquePackageLabels call, where I also send the same CartonIds in. It works just like it should except it uses for CartonId integers starting at 1, even though I provide a CartonId like 10053017. It should be on the shipping label, but instead I see under the barcode text like FBA140CDHXKXU000001, which is the ShipmentId followed by a a single character like 'U' then followed by a 6-digit CartonId (000001). Is it possible to get the CartonIds to come back in the label? The docs suggest it should work that way, but the only FBA images I've discovered have the 00001 type pattern.
Amazon retains your submitted CartonId(s) but does not use them in the labels. All cartons are numbered starting 1 and the format is U.
For example, a shipment (ShipmentID = FBA13H23WS7C) of 2 cartons would generate labels with this text:
FBA13H23WS7CU0000001
FBA13H23WS7CU0000002
When you request unique labels, you do have to pass your CartonId, otherwise it fails.