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.
A piece that is smaller than a Letter-size piece will qualify as a Postcard-size piece which can only be mailed at first class rates
For a complete sizing summary check out our Sizing Summary Page