USPS Flat Rate Envelope | Overview

A Priority Mail Flat Rate Envelope is one of the United States Postal Service's Flat Rate packages which always ships for the same rate within the USA as long as your order fits inside the packaging. The flat rate envelope is intended for documents; if you want to ship something that isn't a document with USPS Flat Rate service, a Small Flat Rate Box may be more suitable.

Where to Find a Flat Rate Envelope

Flat Rate envelopes can be found at your local post office or ordered free online at

Dimensions of Flat Rate Envelopes

  • Flat Rate Envelope - 12-1/2" x 9-1/2"
  • Legal Flat Rate Envelope - 15" x 9-1/2"
  • Padded Flat Rate Envelope - 12-1/2" x 9-1/2"

Rates for Flat Rate Envelopes

The cost to ship a flat rate envelope varies by:

View USPS Flat rate envelope rate comparison table.

Restrictions for Flat Rate Envelopes

Flat Rate Envelopes are perfect for sending documents.

If you need to send items other than documents, make sure that the items do not expand the envelope in any way. When sealing a Flat Rate Envelope, the container flaps must be able to close within the normal folds.

Also, shipments must weigh less than 70 lbs.

More questions? Check out USPS' DMM Regulations for Flat Rate Envelopes.


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