I suggest you write a short, polite letter to the USPS Consumer Advocate/Vice President asking that your Post office enforce its own mailing regulations. Explain what happened and what the correct regulations are.
I did this with UPS when our local service counter started inventing regulations and it worked wonders.
Never have had a problem with FedEx. I use an online account, print my own labels, follow the correct procedures, get billed on my credit card and never say anything to the counter clerks except "have a nice day."
USPS
Consumer Advocate
475 L'Enfant Plaza SW
Washington DC 20260-2200