This class of mail must be within the following dimensions:

If it has an aspect ratio (length divide by height) of less than 1.3 or more than 2.5 it is non-machinable, and will get a higher postage rate. (A square envelope has an aspect ratio of 1, making it non-machinable).

Placement of the address and the address bar code can also affect postage rates. Mailrite’s Account Managers can help you with any questions about Mail Piece design to get the best impact and postage rates. The following USPS link also has detailed instructions on mail piece dimensions: http://pe.usps.com/text/qsg300/Q201a.htm