Assuming the gunsmith is an FFL, then Jeepkid has it right - but I'll add, a handgun can't go USPS, it has to go FedEx/UPS or other carrier. Box it up, unmarked & not shaped like a gun, and ship it. Long guns can go USPS.

Personally, I don't ship mine in their original boxes, because even from the best smiths, sometimes not everything makes it back. I usually buy a cheap plastic handgun case and bed that into a standard shipping box with bubble wrap.