With UPS, yes you have to declare a firearm. Federal law requires that a common carrier be notified as to contents being a firearm. With USPS that's not necessary, as 351 Winchester said, they don't care what's in the package. USPS commonly carries very valuable merchandise like diamonds, precious metals, cash, and the like by registered mail. A few years back, everything including cash being shipped between banks and Federal Reserve Banks went by mail. There wasn't any other way except Railway Express and trains didn't go everywhere. The Post Office did.