Before deciding to mail your piece as a flat, we recommend finding out if it is possible to fold your piece to Letter Size. This will allow you to save money by mailing at the lower Letter price.
Contents must have a uniform thickness with no more than 1/4″ variance starting at 1″ from the edge of the envelope.
If the item is mailing First Class, the mail panel can be placed on any part of the mail piece. However, if the item is mailing Standard or Non Profit, the mail panel must be located with specific quadrants to meet USPS guidelines.
If the spine is vertical, then the mail panel must be within the top half of the piece.
If the spine is on the bottom, then the mail panel must be either on the left or right side of the piece. It cannot be in the middle.