If it were me, I"d make darn sure of the date of manufacture of the gun, and then look up the cut off date for antiques.
In my dealings, I've never had a firearm go USPS--If I'm not mistaken, modern long guns may be shipped USPS, but not modern handguns. The USPS may have policies with regards to shipping any handgun in general, but I don't know. If I'm wrong guys, please correct me. I have no idea about antiques. I feel sure somebody will be along shortly that has more knowledge than me....