i believe it is ok for a non-licensee to shhip a firearm back to the manufacture for repair direct without going through an FFL. Also, I think you can send it 2nd day air, they changed that rule several years ago.
I've used their uppers, parts kits, and lowers. Not had a problem yet. Their uppers are great. Any ammo has worked fine in them. just got 5 more lowers, problem now is finding lower parts kits.
JP