What is the difference between a Letter-Size mail piece and a Flat-Size mail piece (and why is this important)?
-A Letter-Size mail piece is defined as being at least 3 ½” x 5″ in size, but no larger than 6 1/8″ x 11 ½”, and is uniform thickness. -A Flat-Size mail piece is defined as being larger than 6 1/8″ x 11 ½”, but no larger than 12″ x 15″, and is uniform thickness. -Letter-Size mail pieces generally qualify for postage costs that are much less than Flat-Size mail pieces.
