I was refused once & followed this advice. Shotguns and rifles are mailable subject to
specific rules. Best pratice is to print the appropriate sections and have them with you when you deliver the package. If refused be polite and ask for the supervisor . That’s all it took for my package to be accepted. Here’s the link
https://pe.usps.com/text/pub52/pub52c4_008.htm
My opinion USPS a much better bet than UPS, don’t let one clerk chase you away.
William