Looking through the API documentation, there doesn't seem to be a good way to go from TBA number (if you can't just cut off those first three letters) to Package ID.

My order of operations on fixing this problem:

1. Chop the first three letters off the TBA variable you have, convert to integer, try it. Per Andrew Morton's comment.

2. What AfterShip may also be doing is going from the order ID. If the TBA is closely related to the order ID, the Amazon API will give you the information to go from Order ID -> Fulfillment Shipment -> Fulfillment Package -> Package ID. You could then use the Package ID to get your package information. So I'd look at Order ID's as well as Package ID's to see if you could convert one to another.

Given Stevish's comment, it's possible that the TBA can be cut off the TBA number and used as a package number if there is only one package in the order, but things get more complicated in other situations.

If you're working on a site that has the seller Order ID stored on your side, that seems to be the intent they have for getting it through the API.