The USPS two flat rate shipping services:

Prepaid Forever® Priority Mail Flat Rate® Packaging is also available at the post office online store. Enter the measurements of your package to compare the rates given by the calculator with the USPS Flat Rate Boxes and Envelopes listed in the table below: USPS Flat Rate Pricing Table Quickly determine if shipping by USPS Flat Rate is right for you by heading to the USPS shipping calculator on our home page. This service is helpful in that it simplifies shipping decisions for those who are less familiar with typically complicated shipping charges, but it may not always be the best deal, especially when you can get discounted Commercial Rates by using shipping software like ShippingEasy or The caveat is that the item or items you are shipping must fit into a pre-defined box provided by the United States Postal Service (USPS). The Flat Rate shipping that is offered by the United States Postal Service is a service assigns a shipping price based on the box that used no matter what the weight of the item is (if it is under 70 pounds). Flat rate shipping is convenient, but may not always be the best deal.

Ups flat rate boxes