Where to start? Perhaps it’s best to consider what the Post Office has to say. All mailis divided into two primary shapes: “letter-size” and “flats”. Letter-size can be as small as 3.5” x 5” and as large as 6.125″ x 11.5″ with an aspect ratio of 1:1.3 – the ratio of the height to the width. In essence, it is always rectangular. A mail piece is considered a “flat” if either of its dimensions are larger than letter-size.
The most common dimensions for letter size are 4.3” x 9.5” (actual number 10 letter size), 5” x 7”, 5.5” x 8.5” and 6” x 9”. Besides letter size and flats, another type of mailing is called “dimensional.” Dimensional pieces may be either letter or flat size, but they have an unusual characteristic, such as an insert that causes them to be somewhat asymmetrical. Think of a small box or an envelope with refrigerator magnet inside.
Postage for letter-size mail is determined by piece rate. Postage for flats is determined by piece rate plus a weight rate calculation. The piece rate is determined by the number of pieces going to a specific area and how correct the address is. A proper address will be coded by the Post Office (or your mailing house) to a complete zip+4. A “specific area” might be many pieces to the same 5-digit zip code, or the first 3 digits of the zip code. Postage for dimensional mail will be determined by correct addresses, number of pieces to an area and a surcharge.