I have found out that in order to stay competitive the USPS is using a variety of rates and ways to send items. For instance if you used eBay's shipping label and postage it is one rate, going to the window clerk is another rate and using their automated postage machine is still another. Don't you love it? If you send an item in an envelope it must of less than a certain thickness or you should be paying for an oversized envelope postage and if the envelope is RIGID it adds to postage. I have learned this the hard way and asked a lot of questions. The USPS were great in answering questions and for smaller and lighter stuff are still the cheapest. Although they have a web site for postage, I have found it easy to call when I am in doubt and it has saved me money and postage due inconvenience.