Dimensions: A mail piece that is square or has an aspect ratio (length divided by height) of less than 1.3 or more than 2.5 cannot be mailed at automated rates and will be surcharged when mailed at regular rates.
Minimum size: A piece smaller than 3.5" x 5" is not mailable.
Staples: Staples in mail pieces are discouraged because they can be caught in postal equipment and damage the piece and/or the mailing equipment.
Wafer seals: All letter-sized, folded self-mailers mailed at automation rates must have the open (non-folded) side secured by a USPS-approved method (usually tabs or wafer seals). When the open side is at the bottom of the piece, two tabs must be used and they must have a paper (ink absorbent) surface. If the open side is at the top of the piece, one wafer seal may be used. Minimum paper weight for a single folded sheet is 28# (80# text); minimum paper weight for two or more folded sheets is 24# (70# text).
Minimum size: 3.5" high x 5" wide.
Minimum thickness: .007" (7 point) if not more than 4.25" high x 6" wide, or .009" (9 point) if more than 4.25" x 6" wide.
Maximum for cards at card rates: 4.25" high x 6" long with a maximum thickness of .016" (16 point).
Maximum for letters and other cards: 6.125" high x 11.5" wide x .25" thick.
Optical character readers cannot read certain colors such as PMS 100, 101, 102, 106, 107, 133, 114, 120, 127, 128, 134, 155, 379, 386, 394, 461, 587.
To accurately calculate postage weight, weigh 10 pieces and divide by 10. Remember, ink and coating will add weight.
Always consult the United States Post Office for current regulations: http://pe.usps.com/text/dmm100/intro.htm