Ready to give up on shipping parts (or anything)...

Went to the local PO to get a flat rate envelope to mail some hinges to a member. Seems they don't carry flat rate envelopes? Clerk tells me I can use one of the regular envelopes and send it priority. OK. Shock comes when I'm informed that the rate will be $19.80 + the envelope instead of $7.55 like the flat rate envelope. Seems like I always get the same PO'ed woman every time I go in. Maybe if she had been in a better mood she would have told me to use a medium flat rate box for $14.35 since they wouldn't fit in a small one (or if I mailed enough I would have known). If it wasn't such a pain to get to the PO I would have just left and found out on my own.
Not the first time this PO has screwed me over. They have gotten me twice and charged extra when I dropped off a package to ship, saying I weighed it light. I ALWAYS weigh my packages on 2 different digital scales and always come up with the same weight on both. Rather than argue (always a line behind me) I just pay up and leave.
Anyway, rant over. I did send an email to the PO but do not expect an answer.