Print postage from Paypal at home, problem solved. :)
If I would have argued with the cashier or not would probably depend a lot on the price difference. After that, it would depend on how long the line was, how I was feeling, etc etc.
I have pushed things before, going back and forth between the cashier and the manager guy there. I had printed out international postage from home (Canada), but it would not let me insure it online. The USPS site said to purchase the insurance separately at the PO. The cashier and manager guy both said no, they could not do that. I said I wasn't leaving until this package was insured and on its way to Canada. The items inside were pricey and it was curriculum, so the package itself was heavy and expensive to ship. They figured out a way.