The topic of shipping USPS (US Mail) comes up from time to time and there is some misunderstanding of the law, in my humble opinion.
It is legal for a dealer/manufacturer/ffl to ship a handgun or long gun (rifle) through the US Mail to another dealer/manufacturer/ffl as long as he shows proof that both the sender and receiver are eligible under the Post Office rules, no ammo is shipped, the firearm is declared and a special form (PS 1508) is completed, and the firearm is sent registered mail.
Now before anyone starts saying I don't know what I'm talking about, I just returned from my Post Office where I shipped a Ballester Molina to another C&R ffl. Had a long talk with the Post Master (manager) at the Post Office...he pulled out the regs and we read them together, he gave me the proper form to complete, I showed him my ffl and the receiver's ffl and he said it was well within the law and USPS regulations to ship by US Mail.
So, anyone here can say it's not legal...but according to the US Post Office and their book of regulations and their shipping form specifically designed for the shipment of firearms, it is legal. If your local Post Office says no, you should challenge them and have them pull out their book of regulations on shipping firearms and ask about Form 1508.