You can legally ship a rifle through the US Post Office as a non-FFL person. I've done it a couple of times. Some ship in a cheap plastic rifle case covered with cardboard. I've sometimes detached the rifle's action from the stock, wrapped them separately, then they will fit in a shorter box. My local PO demands that you let them check the rifle to see it is unloaded, then seal it up. Funny, it is illegal to bring a firearm on to the premises of a USPO, yet it is legal to ship it thru them. A FFL can legally ship a pistol using the USPS. It makes sense to insure your shipment.