i recently sold a rifle at auctionarms, and have a 2nd up for auction now. when i shipped the rifle i shipped it USPS, about a week and a half ago... rifle was in original remington box, bolt removed, then placed into an unmarked cardboard box with nothing but a shipping label and delivery confirmation label. I had the signed in red ink FFL license from the dealer it was going to in PA. I informed the girls at the post office that it was coming and they were very apprehensive but broke out the book and it is very vague... print off the material you found here.. or at the ATF website and take it along, i had documentation to prove my legality to ship via USPS. I was kind enough to show the post master that the gun was unloaded, the bolt was removed and there was no ammo in the box... we then taped the box shut, put it in the other shipping box and closed it up. once confronted with written PROOF of regulations and the law to combat their ignorance things go pretty smoothly. you may also try a different post office if you get no satisfaction.

hope that helps eh