Gentlemen;
As a stockmaker, it is important to keep ahead of the game. ATF rules state that the out-of-state person receiving the Serial Numbered part of a firearm be an FFL dealer.
You as a private citizen may ship it for repair with in the USA without using an FFL dealer UNLESS your STATE's firearm regulations require it. Shipping the metalwork without the FFL dealer on the other end violates ICC and ATF regulations for the interstate shipment of dangerous goods.
A gunstock by itself DOES NOT require an FFL to ship or recieve. However if you have the metalwork (barreled action, receiver, Pistol frame) in the same package... the craftsman must have an FFL to accept it. The only exception MAY be if the craftsman resides in the same state as the owner. Therefore it doesn't cross a state line via common carrier. One must carefully read your state regulations to be sure. It use to be more simple than this .....but it has really gotten complicated.