All Things Shipping
Feb 10, 2022

USPS Priority Mail vs. USPS Priority Mail Express: Explaining the Key Differences

Shipping with USPS Priority Mail is a standard practice for many e-commerce sellers, thanks to the service’s affordable rates, quick shipping times, and broad availability.

That said, there are actually several distinct solutions under the umbrella of Priority Mail—each of which may be appropriate for your business. Use this article to familiarize yourself with the differences between different USPS Priority Mail services and USPS Priority Mail Express to ensure you’re offering your customers the solutions they want at competitive rates you can afford.

What is USPS Priority Mail?

USPS Priority Mail is the U.S. Postal Service’s standard expedited service. All Priority Mail shipments include USPS tracking and $50 of insurance (some restrictions apply), and are typically delivered within 1-3 business days. 

Priority Mail Flat Rate

USPS Priority Mail Flat Rate offers several specific packaging types and sizes—all of which ship at flat-rate pricing, regardless of their weight.

The current Flat Rate offerings include:

  • Flat Rate Envelope (12 ½” x 9 ½” )
  • Window Flat Rate Envelope (10” x 5” and 12 ½” x 9 ½”)
  • Small Flat Rate Envelope (10″ x 6″)
  • Padded Flat Rate Envelope (12 ½” x 9 1/2″)
  • Legal Flat Rate Envelope (15″ x 9 1/2 “)
  • Small Flat Rate Box (Outside: 8 11/16″ x 5 7/16″ x 1 ¾”; Inside: 8 ⅝” x 5 ⅜” x 1 ⅝”)
  • Medium Flat Rate Box, Top Loading (Outside: 11 ¼” x 8 ¾” x 6″; Inside: 11″ x 8 ½” x 5 ½”)
  • Medium Flat Rate Box, Side Loading (Outside: 14″ x 12″ x 3 ½”; Inside: 13 ⅝” x 11 ⅞” x 3 ⅜”)
  • Large Flat Rate Box (Outside: 12 ¼” x 12 ¼” x 6″; Inside: 12″ x 12″ x 5 ½”)
  • APO/FPO/DPO Flat Rate Box (Outside: 12 ¼” x 12 ¼” x 6″; Inside: 12″ x 12″ x 5 ½” )

Flat Rate packaging is available for free from USPS. As with all USPS Priority Mail services, restrictions do apply on the types of goods that can be shipped in Flat Rate packaging. 

Priority Mail (Weight and Zone Based)

This Priority Mail service bases its pricing on both the weight of your package and the distance it’ll travel, based on USPS mailing zones. The distance between your origin and destination zones also affects how long it’ll take packages to be delivered, as demonstrated by the USPS Delivery Map for the postal code 10001 below:

The USPS Zone Calculator can be used to estimate shipping rates for Priority Mail packages.

Priority Mail Cubic Rate

Many e-commerce sellers aren’t familiar with the USPS Priority Mail Cubic Rate service, but it’s a great option for retailers that sell small, heavy items. That’s because Cubic Rate service is based on a package’s dimensions and the distance traveled—not its weight.

Cubic Rate mailing does have some restrictions. Shipments can’t weigh more than 20 lbs, they can only measure up to 0.5 cubic feet in total, and each side of the package must measure less than 18”. The service is also generally restricted to retailers mailing more than 50,000 packages per year, though you can take advantage of Shippo’s scale to access this service—even if you don’t meet the volume requirement yourself. Additional details on calculating a cubic rate are available at 

Priority Mail Express

Priority Mail Express does offer flat-rate packaging, though Weight and Zone Based pricing is also available. Unlike Priority Mail, however, only three sizes are currently available for Priority Mail Express shipments:

  • Flat Rate Envelope (12 ½” x 9 ½”)
  • Legal Flat Rate Envelope (15” x 9 ½”)
  • Padded Flat Rate Envelope (12 ½” x 9 ½”)

And, of course, there’s the issue of cost. As you might expect, faster delivery times result in higher expenses. Retail rates for Priority Mail Express services begin at $26.95, compared to $8.70 for standard Priority Mail shipments. 

Which USPS Shipping Service Should You Choose?

The question of whether shipping with USPS Priority Mail is a better option than using Priority Mail Express ultimately comes down to speed: how important is it for your customers to receive their packages within a day or two?

If you want to get granular, you can either manually calculate the best shipping service based on your package’s weight and destination, or you can use technology to help you decide. If you need to ship something heavy and small, Flat Rate products may be a better value than Weight and Zone Based Services. Or, if you’re shipping within the same USPS Zone, Priority Mail may get your package there as quickly as Priority Mail Express (though it won’t be guaranteed).

Alternatively, you can put the decision in your customers’ hands. Solutions such as Shippo make it easy to offer buyers access to a wide variety of shipping solutions—all at better rates than what you’ll find shipping directly from the Post Office.

Sign up for free to learn more about Shippo’s discounted rates and services.

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Sarah Gage
is a Michigan-based freelance writer covering business, marketing, and technology topics. She is also the owner of Content Conquered, a B2B-focused content marketing consultancy.

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