Steve, Shhhhhh....
I have talked to Postal Inspectors about this. I have never gotten a firm yes or no. Most replies were "That's a gray area. I'm not sure."
Federal Firearms regs. state that as a C&R holder, you can legally use any carrier. Postal Regs. say Licensed dealers. Technically , a C&R is an FFL to deal(exchange , trade or accept ) specified firearms.
I am not telling anyone to violate any Federal or Postal regulation. However, until I am told otherwise by someone in authority. I will continue to use the carrier of my choice.
Ron