I have never had an issue with shipping a muzzleloader at the post office. I ask the first time I shipped one and they told me it was against policy for them to ask the contents. That I only had to answer the two questions on the screen when I handed it across the counter. FedEx and UPS are different stories, both have refused shipments at their main office. UPS said only FFLs could ship with them and pointed out their sign.
My main concern is insurance, I have told them it is an antique and shipped when the destination ask me to use them, but if it's gets damaged I'm sure they would remind me of their policy.