I had that happen once to me and my post office let it go.
I ordered something and the guy put it in a flat rate box, and then covered the entire box with the light/see thru brown packing tape and somehow only paid $4.00 for it...so when I got it, I owed $5. I refused. And failed to understand how the first post office didn't catch it since you could clearly see the flat rate markings thru the tape. I ship a lot of stuff for work and print my own labels at the office and when I drop things off they always make me wait until they can check weight.
Hopefully you can get lucky and not have to pay the difference.
