Rangefinder, I must respectfully disagree.
Here's what happens when you ship a long gun via USPS:
The gun must be boxed in a plain box with no labels.
At the Post Office, you declare the gun, and seal the box with a paper (has to be moistened) type tape. You can seal this up yourself; get a sponge to dampen the back of the tape, though.
The packaged gun is sent Registered Mail, which is the most secure shipping method. You can insure for up to $25,000; the package is accepted, signed for, and assigned a tracking number that you can use online. The sealing paper tape is stamped in multiple locations so that any tampering is shown.
Registered, Certified and Insured Mail is known as Accountable Mail; it is signed for at each step of the shipment, whenever a transfer is made. Registered Mail is stored under lock and key, too.
Finally, (as an example) it cost me only $28.00 to ship my M1A to Springfield Armory. I had to ship the rifle back to them twice, each time at the same price.
Shipping USPS is the best and most secure way to ship, IMHO--at a fraction of the cost of the other services.