In this case, you would have violated a postal regulation, not a BATFE federal law.
I have had my 03 C&R for many years and never used it to ship a handgun via USPS. Several months ago, I read, and re-read the "newest" USPS regulations and found them ambiguous at best. In one section it mentions C&R and sounds like it is OK as they refer to BATFE regulations as the basis for their regulations. Yet in another paragraph, they mention the 1899 clause.
So.............. I called and spoke at length with a BATFE agent who was familiar with what a C&R actually was
We looked over the regulations, both theirs and USPS. Discussed both sides, discussed both ambiguous paragraphs and came to the following conclusion.................... If you use USPS to ship or receive a C&R eligible handgun, you will NOT be breaking any federal laws and they will have no cause to charge you. BUT, if an inspector at USPS happens to become aware of the package (scan, opened, etc), you can be subject to USPS investigation and charges. What those charges (if any) would be, we had no idea, because technically, the USPS is a privately run business with .Gov oversight.
Confused yet? Good, so were we.
We both agreed that the USPS is contradictory to itself and depending on who you ask, you will get a different answer.
Finished up the conversation with........... Probably best to use an 01 FFL to transfer thru for the extra $15. Paying $15 for a transfer sure as heck beats out trying to fight whatever made up ridiculous charge a USPS investigator can come up with. Personally, I have a great local FFL and have sent and received numerous firearms using them as a go between. Makes life much simpler.