Licensed dealers can use USPS to send handguns back and forth to each other, but not to the end customer. Not even to a licensed collector. HOWEVER, if the pistol is really an antique, the feds do not consider it to be a gun and this was probably legal. (doesn't mean you might not have been arrested anyway)
added: Notice the punctuation in the regulation posted above. Restrictions 1 and 2 only apply to replicas, not actual antiques. So a 1st generation Colt made before 1899 is still an antique even tho' it uses a modern centerfire cartridge.
"The only way to stop a bad guy with a gun is with a good guy with a gun"