How Weight Affects Prices
The weight of your mailpiece is an important factor in determining the postage price you pay. In general, the heavier the mailpiece, the more it costs to mail.
For Priority Mail Express, the weight and distance the piece travels determines the price, except for items mailed in USPS-provided Priority Mail Express Flat Rate packaging. Lower prices are available when postage is paid through Click-N-Ship, authorized PC Postage products, permit imprint vendors, and when USPS Corporate Account, or Federal Agency Accounts are used.
For First-Class Mail, shape and weight will determine the price.
For Priority Mail, the price is a combination of weight, size, and how far the mailpiece is traveling. Priority Mail pieces weighing less than 20 pounds and measuring more than 84 inches, but 108 inches or less, in combined length and girth are charged the price for a piece weighing 20 pounds. This is commonly referred to as a "balloon price." The balloon price applies to pieces addressed for local delivery and to zones 1-4. Priority Mail pieces that exceed one cubic foot, and are addressed to zones 5-8, are subject to "dimensional weight pricing". In general, if a piece is large and relatively light for its size, it is subject to dimensional weight pricing. Items mailed in USPS-provided Priority Mail Flat Rate packaging are charged the same flat-rate regardless of weight (up to 70 lbs.) and domestic destination. Commercial prices are available for postage paid through Click-N-Ship, and authorized PC Postage vendors.
For USPS Marketing Mail, the weight of the letter would depend on the sort level. Carrier Route, Automation, and Machinable Nonautomation letters can weigh up to 3.5 ounces and you pay the same postage whether your piece is 1 ounce or 3.5 ounces. For Nonautomation Carrier Route and Nonmachinable letters, the weight can go up to 4 ounces. Anything over 4 ounces would be classified as a flat-size mailpiece.
For USPS Retail Ground, Parcel Select, and Bound Printed Matter, weight and How distance affects prices determine the price.
For Library Mail and Media Mail, the price is determined only by weight, regardless of how far the mailpiece travels.