We're based in Australia so obviously we don't have any direct advice to offer or an even basic understanding of how USPS works, but if you have the time and inclination it might be worth taking a couple of 'hypothetical parcels' into your local PO and see if you can figure out a reason the website quote might be so different from the actual cost.
Our Australia Post website is a bit dodge as it creates steps for each postage charge, such as up to 250g, 250g to 500g, 500g to 1kg (all really good and generally quite accurate) but then goes 1-2kg, 2-3kg etc. which is where it starts losing all meaning. It would be nice to have a 1.25kg, 1.5kg etc. bracket I think, esp. with international parcels.
Obviously if you can gain a basic understanding of how the postal system actually works you can tell people an approximate cost to what their parcel might be, and that you'll happily refund extra charges.
