Note: Flat Rate prices are available when material is sent in a USPS-produced Flat Rate Envelope or Box. When sealing a Flat Rate Box or Flat Rate Envelope, the container flaps must be able to close within the normal folds. 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.
Weight restrictions for domestic Express Mail and Priority Mail Flat Rate Envelopes and Boxes are 70 pounds.
Weight restrictions for Priority Mail International are 4 pounds for the Flat Rate Envelopes, Small Flat Rate Box, and DVD Box and Large Video Box; 20 pounds for the Medium Flat Rate Boxes and the Large Flat Rate Box.