What manufacturer? The company with the best customer service in the country, Glock, will accept your gun directly from you and return it directly to you. No paper work needed.
Any FFL whos lic includes Gunsmithing can recv a firearm for repair.
The FFL that does the repair can then return the firearm straight back to the owner.
There is no need for a 4473 or NICS check as there is no transfer in ownership.
Even if you chose to have an FFL send it to the Mfg'r, who then returns it to that FFL,,then back to you...No 4473/NICS when you take the gun back as no transfer of ownership was done nor intended (as would be in a consignment for example).
The FFl/Dealer simply farmed out the repair to the Factory. It wasn't ever anything more than that. A gunsmithing Repair.
But don't be surprised if the FFL/Dealer demands that a 4473/NICS be done anyway. His/Her shop,,their rules. Just like demanding the same be done on Antique Status firearms.
If you have a Glock and contact them for service and Glock can send you a pre-paid shipping lable so you can pack and ship the gun yourself with whatever shipping concern they chose to use,,that's fine.
Then Glock can return the pistol right to your address as well. No need to go thru an FFL, unless there is some State or local law demanding it.
If the Company won't issue you a prepaid shipping lable and says it's on you to pack, and then go to one of the big 3 to ship it out & pay the shipping,,then you are stuck it seems these days.
USPS won't ship handguns from non-FFL's
UPS and Fed_Ex have the recent insiders club plan that you have to join as an FFL to be able to ship firearms.
So it's off to a friendly FFL in hopes they will ship it USPO Priority Flat Rate and it won't break the grocery budget.
Boxing it up and shipping it yourself at the USPO and declaring it as a socket wrench set is done by some. But that's on you of course if they check the box addressed to 'Glock' and find the undeclared firearm inside.