The words "envelopes" and "flats" are used interchangeably at USPS when discussing mail that is slightly larger than regular, letter-sized mail. All large envelope/flats must be rectangular or square-shaped and somewhat flexible. You can find out how USPS determines flexibility by referring to their guide.
For your mail to be considered a large envelope/flat and not a letter, it must exceed one or more of the letter-size minimum dimensions listed in the table below.
For your mail to be considered a large envelope/flat and not a package (parcel) it must not exceed the following criteria:
The maximum weight for large envelopes is 13 ounces. Large envelope/Flats can be shipped via First Class or Priority Mail, however a large envelope/flat exceeding 13 ounces is classified as a Priority Mail item.
NOTE: Large Envelopes/Flats shipped via First Class are not eligible for tracking, however, they will include tracking when shipped via Priority mail. Learn more.
For more information, see the USPS page on letters and flats in the Domestic Mail Manual.