If you're seeing the below error on your eBay orders that were booked in with APC, it could very well be an issue with the consignment number that Shiptheory is returning as eBay require the full consignment number gets returned rather than the shorter version.
Could not upload tracking information for Order # XXXXXXX
eBay said: One or more tracking number(s) in the request already exists either for a different buyer or for a different seller.
If this is the case, I recommend following this guide which will show you how to return the longer version to your channel.
If you have any questions about this feel free to get in contact with support or give us a ring on 0117 403 4313