If an envelope is 'oversize' there is a 11¢ surcharge if the item is less than 1 ounce. Over an ounce there is no surcharge.

Oversize refers to both size and thickness. Most POs have a sample slot to drop mail in to test for thickness. If it falls through -- no surcharge. Large, unpadded envelopes also get surcharged. Off hand I don't at what size the surcharge kicks in.

In the real world it is hit or miss whether they catch you or not, hence the reason some padded envelopes get through with only 34¢ on it.