What is the Large Envelope/Flat?

The words "envelope" and "flat" are thrown around a lot in USPS’ verbiage, but they’re interchangeable terms for the same thing. They refer to a package that is:

  • One dimension greater than 6 1/8" high OR 11 1/2" long OR 1/4" thick.
  • No more than 12" high OR 15" long OR 3/4" thick.

Or in other words, "envelope" and "flat" defines most envelope type packages that USPS accepts.

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