325 Mail Preparation
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. Any matter mailed in USPS-produced Critical Mail flat-size packaging will be charged Critical Mail flats prices only if all applicable standards in 323 are met; otherwise such matter will be charged the Priority Mail Commercial Plus Flat Rate Envelope price.
The sender's domestic return address must appear legibly on Priority Mail and Critical Mail pieces.
The marking "Priority Mail" must be placed prominently on the address side of each piece of Priority Mail. USPS-produced Critical Mail envelopes for flats bear the marking "Critical Mail" and must be used for Critical Mail flats. See 302.3.1.
To use a permit imprint, the pieces must be of identical weight and, unless all the pieces are in a weight category for which the price does not vary by zone, the pieces must be separated by zone when presented to the Post Office, except under 3.2.
A permit imprint may be used for mailings of nonidentical-weight pieces only if authorized by Business Mailer Support at USPS Headquarters.
When mailing 200 or more Critical Mail flats in one mailing, prepare Critical Mail flats in USPS-provided flats trays with green lids, place the mail with addresses facing upward in the same direction. Place pieces in trays to maintain their orientation. The weight of a tray and its contents must not exceed 70 pounds. Cover each tray with the green side of the lid facing up and secure the lid with two straps placed tightly around the width of the tray. Label trays under the applicable flat tray label standards in 335.4.0 and as follows:
a. Line 1: Use L201; for mail originating in ZIP Code areas in Column A, use "MXD" followed by city, state, and 3-digit ZIP Code prefix in Column C (use "MXD" instead of "OMX" in the destination line and ignore Column B).
When a manifest mailing system is used, Critical Mail and Priority Mail may be entered on the same postage statement, but mailpieces must be presented separately and may not be combined or commingled in the same container.