At this time the United States Postal Service (USPS) is checking all first class mail to see if it is thick enough for the delivery confirmation eBay and PayPal add to the printed postage option. The envelope must be at least 3/4 inches thick. If your package doesn't make that grade and you send it through USPS your customer may be charged for Priority Mail. It's easy to make that package a little thicker by adding 3 or 4 styrofoam peanuts. They weigh nothing and will definitely 'thicken' your package so you can use that all important delivery confirmation.