I misunderstood, I thought you were using Flat Rate packaging. Using just Priority rates on regular packages always has wildly different results depending on the exact weight and destination. If your package weighs 1 pound and 1 oz, you pay the 2 pound rate.
They should weigh each and every package, if they are not flat rate. If something appears to not have enough postage on it and it gets past the accepting post office, the receiving post office is supposed to either charge the recipient or return your package to you. I was surprised how many letter carriers pay the short postage for their customers and they never know they owed money!
Enough on the post office...I will upset everyone!
Gaijin