What is going on here?  I print prepaid postage and then take the box to the post office and request they weigh it to make sure the postage is correct.  Every time I'm told it's ok but then I end up reimbursing the person receiving the package when they are told the more money is due.

Does anyone else have this problem?  What is the best way to avoid this?

Thanks for any and all tips

When you say "always," how many times has this happened to you?

Make sure you have Media Mail printed somewhere on the package, or the PO will charge for First Class.

Also what is the point of paying the printed postage fee if you are taking them to the PO to be weighed anyway?  My guess is they are not marked the right class that you are sending although if you adjust it to the correct weight it should print Media Mail or First Class. Are you cutting that part off?  Or are you recycling priority boxes/mailers like someone else said?  Are you recycling mailers that say first class but sending them media mail?