A Letter is Not a Letter
While it’s true that a letter is typically correspondence of some type that comes neatly folded into an envelope, the USPS standards are such that a letter can be a brochure also known as a self-mailer a postcard and of course a letter in an envelope. According to the USPS, letter dimensions cannot be smaller than 4¼” x 6,” which may be a postcard or larger than 6 1/8″ x 11½” which would be a flat.
Maximum for letters and other cards: 11½” long, 6 1/8″ high, and ¼” thick. Rectangular, with four square corners and parallel opposite sides. Letter-size, card-type mail pieces made of card stock may have finished corners that do not exceed a radius of 1/8.” Postage cost for each first-class letter ranges from $0.46 per piece (retail) to as low as $0.36 per piece depending on the density of your mailing list. In order to achieve a rate per card less that $0.46 per piece, you would need to mail a minimum of 500 pieces and have your mailing list postal presorted. Postage cost for each standard piece ranges from $0.282 per piece to as low as $0.189 per piece depending on the density of your mailing list. In order to achieve these rates you would need to mail a minimum of 200 pieces and have your mailing list postal presorted.