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All Things Shipping | March 23, 2018

USPS Flat Rate Box Sizes, Prices, and How it Works

USPS Flat Rate Box Sizes, Prices, and How it WorksUSPS Priority Mail Flat Rate boxes are an easy-to-use, cost-effective shipping option. You can easily ship like the experts; just know the USPS Flat Rate box sizes, prices, and how it works before you buy.

USPS Priority Mail Flat Rate boxes are an easy-to-use, cost-effective shipping option. You can easily ship like the experts; just know the USPS Flat Rate box sizes, prices, and how it works before you buy.

How Does USPS Flat Rate Shipping Work?

The USPS Priority Mail Flat Rate service will ship your package anywhere in the U.S. in 1-3 business days for a flat fee. To ship with this service, you’ll first need a free Flat Rate box from the USPS. You have to use the branded boxes to get the Flat Rate shipping price, and you can order them online or pick them up at your local Post Office.

Be careful which box or envelope you select, since each one ships for a different price. The smaller boxes and envelopes are typically cheaper than the larger ones, with some exceptions, so you’re better off with a box that fits your items and packaging materials snugly.

Since the cost of other shipping services will vary based on a package’s destination and weight, the USPS Flat Rate boxes are a great option for items that are traveling long distances or are particularly heavy. Keep in mind that the maximum weight is 70 pounds. And, when sealing a Flat Rate envelope or box, the container flaps must be able to close within the normal folds.

What are the 2020 USPS Flat Rate Box Sizes and Prices?

As you’ll see below, Shippo offers its users discounted prices to what you can expect at your local Post Office. To sign up for Shippo, visit If you have any questions, feel free to contact sales.

2020 USPS Priority Mail Flat Rate

Shippo vs. Retail Rates

ProductsDimensionsShippo Rate Retail RateSavings Using Shippo vs. Retail

% Savings with Shippo

Flat Rate Envelope12 ½” x 9 ½”$7.15$7.75$0.608%
Legal Flat Rate Envelope15” x 9 ½”$7.45$8.05$0.607%
Padded Flat Rate Envelope12 ½” x 9 ½”$7.75$8.40$0.658%
Small Flat Rate BoxOutside:

8 11/16” x 5 7/16” x 1 ¾”


8 ⅝” x 5 ⅜” x 1 ⅝”

Medium Flat Rate Box – 1 (Top Loading)Outside:

11 ¼” x 8 ¾” x 6”


11” x 8 ½” x 5 ½”

Medium Flat Rate Box – 2 (side-loading) Outside:

14” x 12” x 3 ½”


13 ⅝” x 11 ⅞” x 3 ⅜”

Large Flat Rate BoxesOutside:

12 ¼” x 12 ¼” x 6”


12” x 12” x 5 ½”


Are there any other restrictions?

There are specific regulations and standards for mailing restricted, prohibited, and hazardous materials, including cigarettes and smokeless tobacco. You also can’t ship airbags, ammunition, explosives, or gasoline. For more details, visit the USPS Shipping Restrictions page.

How do I get a USPS Priority Mail Flat Rate Label?

It takes just a few minutes to set up a Shippo account, and then you can print the label from your office or warehouse. Affix the label to the USPS Priority Mail Flat Rate box. Then, put it in your mailbox or schedule a pick-up with your local mail carrier in the Shippo dashboard.

Make sure the label you’re using matches the packaging. For example, if you are using a Priority Mail Express Flat Rate box, you need to use a Priority Mail Express Flat Rate label.

If you liked this post, you might also like our comprehensive blog post on the 2019 USPS shipping rate charts. You may also be interested in a recent video that our co-founder Laura Behrens Wu did with the co-founder of packaging solution company Lumi on shipping with the USPS.

Shippo is a multi-carrier API and web app that helps retailers, marketplaces and platforms connect to a global network of carriers. Businesses use Shippo to get real-time rates, print labels, automate international paperwork, track packages and facilitate returns. Shippo provides the tools to help businesses succeed through e-commerce shipping.

Michelle McNamara is a Senior Marketing Manager at Shippo

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