Mailing Standards of the United States Postal Service - Domestic Mail Manual > 100 Retail Mail Letters, Cards, Flats, and Parcels > 120 Retail MailPriority Mail > 125 Mail Preparation
2.0 Marking for Priority Mail
Priority Mail packaging provided by the USPS must be used only for Priority Mail. Regardless of how the packaging is reconfigured or how markings may be obliterated, any matter mailed in USPS-produced Priority Mail packaging is charged the appropriate Priority Mail price.
When sealing a Flat Rate Envelope, Flat Rate Box, or Regional Rate Box, 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.
The sender’s domestic return address must appear legibly on Priority Mail.
The marking “Priority Mail” must be placed prominently on the address side of each piece of Priority Mail under 102.3.2.