The US Store, Kinko's-FedEx, Staples, etc will charge you a king's ransom for the box.
I generally put the gun in a cheap, plastic gun case then get a furniture or appliance box from somewhere and make my own cardboard box to fit the case in. I usually tape bubble wrap around the case before I box it in. UPS doesn't make a habit of intentionally mis-handling packages. But the mechanical damages from moving that many packages that fast can be brutal when they happen. ALWAYS overpack for the worst case scenario.