Scott- In the U.S. if we go to the post office and have a clerk weigh and put a machine stamp on the package it will always have the exact and correct postage. If we put our own stamps on the package and drop in the mail box, anything can happen. Because the package is never weighed, a clerk handles the package and just uses his judgement if correct postage is on package. He can be liberal, and if busy let anything go.So you will see a vairation in postage on the same packages.(bubble mailers) Hope this helps.