Our fulfillment center uses "FedEx Cross Border" a.k.a. "TrakPak" for our international shipments. This is apparently not supported by Shipstation. Here's an example of one of our shipments: https://fcbtracking.fedex.com/?MyFICNumber=GFL0990US55445444001.
In our marketplace, Shopify, we can enter the tracking info using carrier "other", which allows us to enter the tracking link, as well as the tracking number. Using "other" in the Shipstation "mark as shipped" dialog doesn't provide that option.
Short of actual support for FedEx Cross Border, it would be great if selecting carrier "other" in Shipstation would allow us to supply the appropriate tracking link.
We don't do a ton of international shipments, but it's quite a pain to deal with the ones we get.