Mailing Standards of the United States Postal Service Domestic Mail Manual > 100 Retail Mail Letters, Cards, Flats, and Parcels > 120 Retail MailPriority Mail > 124 Postage Payment Methods
1.0 Basic Standards for Postage Payment
Priority Mail postage may be paid with postage stamps (see 604.1.0), with meter stamps (see 604.4.0) affixed to each piece, or with USPS Click-N-Ship.
Priority Mail Forever Prepaid Flat Rate packaging must be purchased by credit card online at www.usps.com and bears a shipping label with affixed Forever Prepaid postage. Mailers adding extra services must take packages to a retail Post Office location for payment of the associated fees for each extra service added. USPS Tracking (electronic option) is included at no additional cost.