Question: what is the difference between letter and flat mail?
Answer: 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.