The cost for sending flat rate priority mail is determined solely by the size of the box or envelope. The Flat Rate mailer measures 12.5 inches x 9.5 inches while flat boxes come in these sizes: small (8 5/8″ x 5 3/8″ x 1 5/8), medium (11″ x 8 1/2″ x 5 1/2 and 13 5/8″ x 11 7/8″ x 3 3/8) and large (12″ x 12″ x 5 1/2). When shipping thru this option, customers will only pay a flat rate depending on the size of the envelope or box, regardless of destination, as long as the package weighs less than 70 pounds. Items weighing more than 70 pounds will not qualify for this service.
Any item can be shipped thru this option as long as it is entirely confined within the envelope/box. Altering the shape and size of the envelope/box to accommodate an item is prohibited. USPS Flat Rate Priority Mail envelopes and boxes are available in post offices free of charge or you can order them at USPS.com. Only packages contained in these envelopes/boxes are qualified for flat rate priority service.
The USPS Flat Rate Priority Mail has become a popular shipping option because heavy items that are small enough can be shipped at cheaper rates. Further savings are made because shipping supplies, such as envelopes and boxes, are free.
On the other hand, pricing for regular priority mail is based on the weight of the package and the distance it travels. Shipping supplies such as envelopes or boxes are also available for free but customers can also provide their own supplies. This options is best for irregularly-shaped items that will not fit in any flat rate priority mail envelopes or boxes.
Extra services, such as insurance, delivery confirmation, special handling and return receipts, can be availed for both types of priority mail services. Package pickup, Saturday delivery and residential delivery are also available for both options at no extra cost.