I return guns and receivers all the time with the USPS, but not pistols; normally using Priority Mail. It is fast, very safe, but insurance is the only drawback.
No one needs an FFL to ships a gun in for work, only when a TRANSFER OF OWNERSHIP takes place, and then it MUST be done from FFL to FFL.
Anyone can send a gun to an FFL for repair work and it can legally be returned to that owner. It would be next to impossible for a gun owner to get, for instance, a copy of Remington's FFL to ship a gun to them for repair. That just isn't going to happen.
I *just* had that issue with a FedEx office in Yuma AZ. They required the owner to provide to them for filing purposes a copy of MY FFL. I faxed them information that (1) they are ignorant of their own policies, (2) they absolutely cannot keep my FFL on file, (3) they were making it very difficult for a customer to ship his Perazzi in for repairs, and (4) I provided them the link to log onto the BATFE site to verify the receiver's (my) FFL.
I also informed an executive friend of mine in FedEx's corporate office about the hassle they were giving to my customer. I've not heard anything back from Atlanta, but I do know that when the customer came back in, they were very cordial to him and his gun was shipped out without any further problem(s).
UPS Stores, not UPS actual facilities normally will not ship guns either. I understand it is their company policy. They take mine, but are careful not to ask me what is in the box. Should it be a problem I just hand it over to the UPS driver.
Whiz