Walmart and a few other discount stores used to sell cheap plastic rifle cases that were in the $15-$25 range that work fine for shipping. If they come in a box, it usually says on the outside of the box what is inside, so I usually turn the box inside out and put the case inside when shipping... I wouldn't trust a cardboard box by itself- too many horror stories about UPS, USPS, or FedEx destroying guns in cardboard boxes. At least with a stiff plastic gun case you have a pretty good chance of it surviving an intentional or otherwise toss or stomp on the outside...
Bob
