- “The Postal Service uses the word ‘flats’ to refer to large envelopes, newsletters, and magazines. The words are interchangeable. However, flats must have one dimension that is greater than 6 inches high, 11½ inches long, or ¼ inch thick. And they must be no more than 12 inches high, 15 inches long, and ¾ inch thick.”
- Instead of a handwritten address, the use of a label is preferred on flats. A label without a return address printed onto it. Using a Sans Serif type font such as Arial or Helvetica with a minimum 9 point font size is preferred.
- Envelopes that have clasps, protrusions (pens, etc.) buttons or CDs will not qualify and will be processed as parcel mail which is processed at a higher postal rate. Use only white paper envelopes for the lowest cost.
The average price difference per a 1 ounce 9″ × 12″ envelope that does not meet the above will cost an additional .51 cents per piece.
See Sizes for Large Envelopes and Flats from the USPS for more information.