Owner can ship to an FFL holder for repair, custom work or return.
Can ship via UPS (over night air their regulation).
Can ship FedEx (don't know of any restrictions)
Can ship USPS (long guns for sure, handguns have had yes and no in different P.O.s' in same town).
Now considering the above I have never had a UPS counter worker ask what was in the package, same goes for FedEx. I usually ship long guns in their boxes so USPS always knows I'm shipping guns.