I'll be honest and probably piss off some people I know that work for the PO but I'd rather try to explain USPS regulations to a pine tree than a postal clerk as I'm certain I would be more successful at the end of my effort.
I don't think those working the desk are as familiar as those delivering the mail.
I did get the big eye when I carried that Medium Flat Rate Shipping box that weighed 43 pounds, to have shipped for $13.80. lol