Using my local FFL pawn shop is a good idea. I will see if he is agreeable. You may have seen this before so I apoligize for repeating but these paragraphs from the USPS regulations make it pretty clear that you can legally use the post office for bp handguns.
11.1 Pistols, Revolvers, and Other Concealable Firearms
g. Antique firearm means any firearm (including those with a matchlock, flintlock, percussion cap, or similar type of ignition system) manufactured in or before 1898, or any replica thereof, if such replica:
1. Is not designed or redesigned for using rimfire or conventional centerfire fixed ammunition.
2. Uses rimfire or conventional centerfire fixed ammunition that is no longer manufactured in the United States and that is not readily available in the ordinary channels of commercial trade.
11.2 Antique Firearms
Antique firearms sent as curios or museum pieces may be accepted for mailing without regard to 11.1.3 through 11.1.6.