The PO clerk runs a quick verbal liner past the customer asking 'if the parcel contains anything liquid, fragile, hazardous,,etc' something of that wording. That covers their Pub52 and within that reg. is 'firearms'. Firearms are USPO listed as a 'restricted' item.
While not specificly asking each individual package contents, that statement/question asked by the clerk brackets all the items that the USPO requires to be declared by the shipper in one short quick question.
Common carriers (UPS/FedEx/etc) are specificly required to be notified of a firearm in contents when the package is placed into their system for shipment according to BATF regs I read, but I don't see such specific wording for USPO shipments.
Neither can force you to lable the outside of the package as containing a firearm. No need for 'dissassembly' or 'the firing pin must be removed'..just a couple of the common responses I've gotten over the years from unknowledgable clerks.
It's rare in my experience to find a USPO/UPS/FedEX/etc clerk that knows the shipping rules anyway, either their own employer's or the BATF & State regs.
So expect anything from a roll of the eyes in disgust of the contents to an outright refusal to ship your package.
It is quite legal w/some restrictions as noted especially when it comes to handguns. A copy of their own rules in hand and a copy of the destinations FFL will help smooth things out.
It can be an adventure to say the least. It gets better over time as you educate them with each trip to the counter.
