Flat (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:

Have one dimension that is greater than 6-1/8 inches high OR 11-½ inches long OR ¼ inch thick.
Be no more than 12 inches high x 15 inches long x ¾ inch thick.
Weigh no more than 13 ounces.

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.