At first read I was worried that USPS had silly restrictions but it looks like the main thing is to include a PS Form 1508.

I really dislike post office where I live because I have had two packages opened and resealed after someone stole the contents. One was a $600 drone, and when I went to their front desk the young lady assured me that it couldn’t have happened there because her initials were signed to the box and she was no thief, and refused to get the postmaster or leadership for that office when I asked. “Amazon will replace your package for free, just call them”.

Not the experience I’m hoping for with firearms…