If an item is an ounce or less and it is over a certain thickness their is a surcharge for mailing it. I also think the surcharge applies if it is over a certain length or width. The exact surcharge I do not recall. If it is over an ounce there is no surcharge. If you cannot find a reference to this surcharge on the USPS web site, the next time you go to the post office look at the posters in the lobby relating to mailing rates. It might be listed there