Postal Standards - The Graphic Design: Reference and Specification - page 120

the tip of the upper-right corner of the
mail piece must touch this area to be
mailed at regular postage.
subject to surcharge
off-size square envelopes and mail not complying with usPs mailing standards are subject to a surcharge. refer to this template
to determine if your piece falls within usPs guidelines for proper size and aspect ratio. (scale of diagram is 50% of original size.)