I always personally drop my pre-paid USPS packages off at my local post office. It's right on the way to other local stops. If I'm in doubt about the dimensions, I will ask the clerk to measure it to see if they agree with my measurements.
I was surprised by one where USPS made an automatic deduction from my paypal account for the difference in postage as paypal is the funding source for my labels.
My post office told me that they accept all prepaid packages even when they can see the weight is wrong or it's oversized without warning the customer. They have been told not to warn the customer as it will be caught later in USPS sorting facilities. They know you may have received a label discount when pre-paying, and if it's actually heavier or bigger than what you paid for they will recalculate the shipping charges without the discount and charge you for the difference. So they are instructed not to warn the customer which would give the customer a chance to void the incorrect label, find a smaller box, and/or print a new label with the correct weight and size still able to get their online discount.
It's kind of annoying, if you walked into a post office with a package that you needed to buy postage for, they will weigh it and measure it and tell you what it costs, but if you walk in with a prepaid package they won't measure it even when they know it's wrong, letting you wait to find out the hard way.
