Hal,
I've come across some bureaucratic clerks in my time but never one as surly as the one in your PO!
When I am faced with a PO that has less than stellar customer service, I usually affix the postage before I get to the counter. I believe they have to use the meter tape regardless for tracking purposes but it makes the clerk calculate the postage balance due.

If I am accurate, the meter reads. $0.00.
The USPS now has "customer feedback" on its web site. You need information from the receipt. It identifies the PO, the clerk and the time of the transaction. Now I'm no Pollyanna. If the feedback has *any* affect, it will be long term. Still, it gives you a place to vent.
Dan
