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The United States Postal Service uses the words "flats" and "nonletters" interchangeably to refer to large envelopes, newsletters, and magazines.
Contents
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 plenty of room to put lots of material in the envelope. An issue, however, is that the higher the weight the higher the postage, especially for First-Class Mail.
References
Flats (USPS) Wikipedia(Text) CC BY-SA