
If you want to send letter-sized papers without folding them, you can use a large envelope (called a "flat") the postage for flats starts at $1.35. 10 envelope is 9-1/2" long x 4-1/8" high.) You can fold what you put in your envelope, but it needs to stay flat-no more than 1/4" thick. Your envelope can be a maximum of 11-1/2" long x 6-1/8" high. You may order free packaging supplies at /store.Envelopes must be rectangular and made of paper to qualify for letter prices. Weight restrictions for Priority Mail International Large and Medium Flat Rate Boxes are 20 pounds, and 4 pounds for Priority Mail International Flat Rate Envelopes and Small Flat Rate Priced Boxes. Weight restrictions for Priority Mail Express International Flat Rate Envelopes are 4 pounds. Weight restrictions for domestic Priority Mail Express Flat Rate Envelopes, and Priority Mail Flat Rate Envelopes and Boxes are 70 pounds.

Tape may be applied to the flaps and seams to reinforce the container provided the design of the container is not enlarged by opening the sides and the container is not reconstructed in any way. When sealing a Flat Rate Box or Flat Rate Envelope, the container flaps must be able to close within the normal folds. Note: Flat Rate prices are available when material is sent in a USPS -produced Flat Rate Envelope or Box.

Designing Letter and Reply Mail (PUB 25).
