I always get the ones I buy thru UPS. I guess if I were going to ship one that was going to be shipped back eventually I would I would buy one of those 40 buck plastic gun cases and ship it UPS or USPS.
If just using cardboard I would wrap it up good in foam or crushed newspaper and use a couple of boards in there too to keep it from getting bent.
If your shipping in PA use the post office.
Pack it good and tight in a plain cardboard box. Don't allow any room for it to move inside box. Wouldn't want a barrel to come poking out during process. Take to PO - they will weight it and ask how you want it sent. I found that Priority mail, with insurance and return receipt works for me.
Mark it as fragile if they ask for the insurance. If they ask if it is a gun you can say yes. If they tell you they can't ship it they don't know what they are talking about. Ask to speak with the postmaster.
Pretty simple process, just like shipping anything else of value.
Need any more help. Let me know. Good luck!