What is USPS Flat Rate and When Should I Use It?

August 1, 2019

USPS Flat Rate is a commonly used shipping service. A major reason it’s sold so frequently is because it’s a very easy parcel size for the USPS to fit into their trucks. It’s also a good way for the USPS to make money. As a result, they’ll usually suggest it when you go into a post office: These parcels rarely hit the weight limit and, frequently, standard Priority Mail Package is usually a more economical way to get the parcel to the customer. Knowing when Flat Rate options offer the best rate and when standard packaging or Regional Rate boxes offer the best rate is important.

How Does Flat Rate Work?

USPS Flat Rate is a type of USPS Priority Mail® packages that are specific-sized boxes and envelopes that can be shipped at one flat fee across the US as long as they weigh under 70 lbs. While they range in price based on size, the boxes themselves are available through the USPS free of charge so long as they are used with the appropriate service. They may not be used with standard Priority Mail shipments, First Class Mail, or Media Mail. Only their designated Flat Rate service.

How much does Flat Rate packaging cost? 

Priority Flat Rate is the USPS’s one-size-fits-all packaging that lets you fit as much as you can into the box as long as it’s under 70 lbs. The prices and dimensions of each package type are listed below. 

Flat Rate Box SizesShipStation price
Envelope: 12 1⁄2″ x 9 1⁄2″$6.95
Legal Envelope: 9 1⁄2″ x 15″$7.25
Padded Envelope: 12 1⁄2″ x 9 1⁄2″$7.55
USPS Flat Rate Small Box: 8 11⁄16″ x 5 7⁄16″ x 1 3⁄4″$7.50
USPS Flat Rate Medium Box: 14″ x 12″ x 3 1⁄2″$12.80
USPS Flat Rate Large Box: 12 1⁄4″ x 12 1⁄4″ x 6″$17.60

What are Regional Rate Boxes? 

Regional rate boxes function similarly to USPS Flat Rate packaging. However, they do increase in price based on the destination zone. As a result, the closer USPS zones are more affordable than flat-rate packaging. Unlike other Priority Mail services, though, Regional Rate Boxes have weight limits of 15 lbs or 20 lbs. The USPS also doesn’t always push these as hard as they do standard Flat Rate packaging, and there can be additional fees if you purchase these from the USPS directly. Because of this, it is advisable to use your own label-creation software like ShipStation.

BoxesZone 1 & 2Zone 3Zone 4Zone 5Zone 6Zone 7Zone 8Zone 9

*Max weight 15 lbs.
Side-loading: 11 1/16” x 2 ½” x 13 1/16”
Top-loading: 10 ⅛″ × 7 ⅛″ × 5″
°Max weight 20 lbs
Side-loading: 14 ½″ × 3″ × 16 ¼″
Top-loading: 12 ¼″ × 10 1⁄2″ × 5 ½″

Priority Package

Priority Package is the standard parcels for the USPS that weigh under 70 lbs and do not exceed 108″ in combined length and girth (girth = the distance around its thickest part). These parcels will reach their final delivery in the same timeframe as the flat rate and regional rate boxes. However, their prices are structured differently. Whereas flat rate and regional rate don’t have weight factored into them. Each new pound increases the rate for standard Priority Package. This also includes Priority Thick Envelope.

USPS Priority Mail Requirements

  • Maximum weight is 70 lbs.
  • Maximum combined length and girth (girth is the distance around the thickest part) is 108″.
  • For more information on USPS pricing available in ShipStation, click here.
Weight Zone1 & 23456789

Can I Get Free Boxes For These Services?

Yes! An added bonus of USPS Flat Rate and Regional Rate boxes is that USPS offers free packaging for these services. For Priority Mail Package and Thick Envelope, you usually have to provide your own packaging. Keep in mind, though, that you can only use this packaging for their associated service. If you try and use a Flat Rate box with a standard Priority service or First Class Mail, you should expect a post-billing correction to be made.


For most parcels under about 10 lbs, USPS Priority Mail will be the best option for shipping. If it is going to a faraway destination like Hawaii or opposite coasts, or you’re shipping something heavy, Priority Flat Rate can be more cost-effective. For shipments not going as far, using Priority Mail Package or Regional Rate Boxes can be the most affordable option.

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