I hope I am not the one owing the postage because I have not gotten any emails. In any case the post office has the worst rules for shipping. The way they may be able to get you is if the package is not "bendable". Unfortunately they do not train the clerk critters on what Bendable means. If it is not bendable then it is a package. As long as it is under a certain size and not more than 1/4" thick it can be considered a letter. 1/4" to 3/4" is a large envelope and anything more than 3/4" thick is a package. Generally the best thing to do is to use the machine if they have one. I have never (knock on wood) had a problem with postage from the machine.