Found a link to this - very specific about firearms and knives, says nothing about suppressors or other parts. https://pe.usps.com/archive/html/dmmarchive20030810/C024.htm
Thanks....I'm not sure. The guy at AAC was very adamant to only use UPS or FedEx as it was a felony to ship via USPS (suppressors that is). He said most go through with out incident, but if they catch it, they'll nail your ass....is what he said.