Flats (USPS)
The United States Postal Service uses the words "flats" and "nonletters" interchangeably to refer to large envelopes, newsletters, and magazines. Size restrictions. To fit the definition a flat must: Furthermore, the item must be somewhat bendable: see the USPS Domestic Mail Manual for exact details. This general rule does not apply to: Automation rate flats and Standard Mail Enhanced Carrier Route flats. Postage. The maximum size for a flat provides enough room to enclose much material. An issue, however, is that the higher the weight the higher the postage, especially for First-Class Mail.