Q. Just how much material can be mailed in a flat-rate envelope?

A. Any amount of material that can be mailed in the special Priority Mail or Express Mail flat-rate envelope available from the USPS is subject to the appropriate 1-pound rate for Priority Mail and the 1/2-pound rate for Express Mail, regardless of the weight of the material placed in the envelope.

However, keep in mind when mailing at the Priority Mail or Express Mail flat rate, the contents of the flat-rate envelope must be confined within the envelope with the adhesive provided on the flap as the primary means of closure. The flap must be able to close and adhere to the envelope. Tape may be applied to reinforce the envelope provided the design of the envelope is not enlarged by any means (i.e., opening the sides of the envelope and then taping or reconstructing the envelope).