I've shipped all my antique firearms through USPS, but I had a lot of trouble in the beginning. First time I tried they freaked out and told me "they don't ship guns of ANY kind."
I went home and called our main post office and got a very nice guy who was sympathetic, and told me to go back and tell them he said it was OK. I went back and gave them his name and number, and they still refused.
Headed home and called him again, he directed me to their web site and the correct area to print out their regulations. I printed out the rules and took them with me. Same lady looked them over and still wouldn't accept it until she got her boss to read them.
They finally accepted the package and made a copy of the rules. I've gone back many times since then, and always keep the rules with me.
But the real key is what the guy at the main post office told me, "Don't tell them what's in the package!"
Seems that those 4 questions they ask are not anything to do with what's inside, and he said if thye asked me what's inside to tell them, "You're not supposed to ask that."
My mistake the first time was the lady casually asked, "What's in here that's so long and heavy?" And I mistakenly told her.
Since then I simply drop the package on the counter, pay the fees, and head home. Ask me no questions, and I'll tell you no lies!