A lot of post offices have a stroke if you try to send a C&R weapon but some are okay with it. I guess it comes down to if the people working there know the regulations.
I spoke to a post master one day about these issues. He stated hat only a 01 FFL can ship a non antique handgun through them. They have no special treatment for C&R handguns. He stated that shipping a long gun is ok as long as it stays in state or goes to a factory or FFL. But even so, they are more expensive than fedex.