The words "flats" and "envelopes" get thrown around a lot when shipping with the USPS. Fortunately for shippers, the two are used interchangeably and refer to the same types of packages. Regardless of name, flats or large envelopes can be shipped via First Class or Priority Mail, and must meet the following criteria:
- Have one dimension that is greater than 6-1/8" high, OR 11-1/2" long, OR more than 1/4" thick.
- Be no more than 12" high, OR 15" long, OR 3/4" thick.
Here's a table to make those bullet points a little more clear!
*Flats/Large Envelopes must exceed one or more of the letter size minimum dimensions listed above.
While the largest flat/envelope can fit lots of material, keep in mind that the postage cost of a flat increases with its weight.
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.
If you're still a little confused, you can consult the USPS page on letters and flats in the Domestic Mail Manual.