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 (the side parallel to the address as read) 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.External links and references