On the other hand, if your firearm is repaired, and returned to you, I believe that they can send it back to you directly, and not an FFL, with a shipper like UPS or FedEx that tracks and requires signature.
My wife's Walther CCP was recalled, and a part updated. We sent it in via FedEx and I think (trying to remember now) that it came back directly to us...
When bought through the CMP, my Garands came directly to the house via FedEx. Six pages of paperwork is done, and the background check is part of it...
In your case, Eosyn, if the frame was cracked (as I think it might have been) then the gun you get is "new" anyway, because the part with the serial number has been replaced. New serial number = new gun.
So, again, for example, you can ship rifle parts or gun parts through the mail, or common carrier, all day long, but the part with the serial number (the receiver for most rifles, the frame for most pistols) is the "gun" and THAT part must go through an FFL.