What Are USPS Flat Rate Boxes? Complete Explanation | ShippingEasy

What Are USPS Flat Rate Boxes?

By Rob Zaleski
Jul 28, 2020
What are USPS Flat Rate Boxes and How Can You Use Them?

Flat-rate shipping is a highly underutilized shipping method because not all merchants know what options are available. We see the question all the time, “What are USPS Flat Rate boxes?” This article is intended to answer that question. From the box sizes available to how the varying flat-rate shipping options can be used in conjunction to save you money, we’ll break down the concept of USPS flat rate boxes for you.

(Note: the shipping costs for the below USPS Priority Mail Flat Rates options have been updated to reflect 2021 shipping costs)

What is USPS Priority Mail® Flat Rate?

USPS Priority Mail Flat Rate allows you to pay a single rate regardless of package weight and distance traveled, up to 70 pounds. So whether your 20-pound package is shipping across town or across the country, the rate is the same.

This service type is offered for both standard Priority Mail shipping as well as Priority Mail Express, and both include standard tracking, up to $50 in insurance, and typically deliver in 1-3 days (including Saturday at no additional charge). You can schedule pickups so you don’t have to drive to the local post office.

Get the best rates around, even on Priority Mail Flat Rate options. Start your free trial of ShippingEasy now!

There are numerous package sizes available, which we will cover shortly. One variation on USPS Flat Rate boxes is the online-only Regional Rate option, which does take distance traveled into account and maxes out at 15 or 20 pounds based on which box size you use. We will also cover that later in this article.

One of the greatest conveniences of the flat-rate package type is that you can order the supplies from USPS for free, and they’ll deliver your order to your home or office free of charge. When you use ShippingEasy, you also get discounted rates on USPS Flat Rate shipping rates. You can see the discounts on standard Priority Mail Flat Rates below.

Priority Mail Flat Rate
2021 RetailSavings
Flat Rate$ 7.40 $ 7.95 7.43%
Legal Flat Rate Env$ 7.70 $ 8.25 7.14%
Padded Flat Rate Env$ 8.00 $ 8.55 6.88%
Small Flat Rate Box$ 7.90 $ 8.45 6.96%
Medium Flat Rate Box$ 13.75 $ 15.50 12.73%
Large Flat Rate Box$ 19.30 $ 21.90 13.47%
APO/FPO/DPO Large Flat Rate Box$ 17.80 $ 20.40 14.61%
*must use boxes

Now let’s cover the various types of packaging you can choose.

What are USPS Priority Mail Flat Rate Boxes?

Perhaps the only downside is that you have to use USPS-branded boxes to take advantage of this shipping method. You cannot use USPS Flat Rate shipping in a plain brown box or your own branded packaging. As long as that is not crucial to your business’s branding strategy, these free boxes are worth keeping in mind. The contents of the package must also fit comfortably inside without any bulging.

USPS Flat Rate Box

As the weight of your package or distance traveled increases, the benefit of using the various USPS Flat Rate boxes also increases. Let’s dive into what USPS Flat Rate packaging is available to use and how each of them can benefit your business.

USPS Priority Mail Flat Rate Envelopes

There are three types of envelopes available in USPS Flat Rate shipping. Some offer a bit more protection than others. The three types are:

  • Priority Mail Flat Rate Envelope: 12 ½” (L) x 9 ½” (W)
  • Priority Mail Flat Rate Legal Envelope: 15” (L) x 9 ½” (W)
  • Priority Mail Flat Rate Padded Envelope 9 ½” (L) x 12 ½” (W)

All of the above are available with the Priority Mail Flat Rate Express option as well. These envelopes are perfect for documents, tickets, art, jewelry, and other non-bulky items. Aside from a few instances of very nearby Zones, these envelopes will likely save you over weight-based Priority Mail shipping options.

USPS Priority Mail Flat Rate Small Box

For smaller items, particularly small dense/heavy items, that won’t fit in an envelope, USPS Priority Mail Flat Rate Small Box is a great option. Below are the dimensions for a Flat Rate Small Box.

  • Outside: 8 11/16″ x 5 7/16″ x 1 3/4″
  • Inside: 8 5/8″ x 5 3/8″ x 1 5/8″

USPS Priority Mail Flat Rate Box

USPS describes the box size as about the same as three stacked DVD cases, for those of us who remember what DVDs are. The boxes feature self-adhesive flaps, which must be able to close fully based on the folds. This indicates that your contents can’t bulge the flaps and must fit fully inside the box. That will be the rule for all of the boxes we will discuss here.

This box type is great for bulkier jewelry, vape supplies, small electronics, and more. For example, our customer Furls Crochet uses Flat Rate Small Boxes to ship their custom crochet and knitting needles.

USPS Priority Mail Flat Rate Medium Box

One major departure from the Small Boxes to the USPS Flat Rate Medium Boxes is that there are two versions. Medium Boxes come in a side-loading version, which is a bit more narrow, and a top-loading version, which offers a bit more inside space. These two versions can be helpful in better accommodating varying sized products.

USPS Flat Rate Medium Box-1 Specs (top-loading):

  • Outside: 11 1/4″ x 8 3/4″ x 6″
  • Inside: 11″ x 8 1/2″ x 5 1/2″

USPS Flat Rate Medium Box-2 Specs (side-loading):

  • Outside: 14″ x 12″ x 3 1/2″
  • Inside: 13 5/8″ x 11 7/8″ x 3 3/8″

You can see the differences between the two below.

USPS Priority Mail Flat Rate Medium Boxes

As you can see, the types of products that would easily fit in these two boxes varies wildly, but they cost the same amount to ship. Box 1 could be used to ship shoes. Box 2 is what we use when we ship dozens of printed versions of our E-commerce Seller’s Guide to Shipping Rate Changes to our partners, for example.

Also, once you reach this box size, it’s more crucial to compare the cost versus weight-based shipping. There are many instances where the latter would still be cheaper than flat-rate shipping depending on weight and destination.

USPS Priority Mail Flat Rate Large Box

The USPS Flat Rate Large Box really offers value once you’re shipping larger, heavier packages. You’ll find this method cheaper than weight-based options for packages weighing more than 25 pounds to most Zones. Packages traveling further across the country will likely be cheaper using USPS Flat Rate Large Boxes depending on the weight.

For scale, these boxes could be used to ship larger electronics like computers and laptops.

  • Outside: 12 1/4″ x 12 1/4″ x 6″
  • Inside: 12″ x 12″ x 5 1/2″

USPS Priority Mail Large Flat Rate Box

A benefit of using a technology like ShippingEasy is that you can set rules for assigning shipping selections. If you have larger items that travel varying distances, you can set rules to select either USPS Flat Rate Large Box or USPS Priority Mail to automate making the better decision based on weight and distance traveled!

USPS Priority Mail APO/FPO/DPO Large Flat Rate Box

This package type is specifically used for sending items to troops and military personnel stationed overseas. It has the same dimensions as the USPS Flat Rate Large Box but includes a message of support for the troops and offers a discounted rate.

  • Outside: 12 1/4″ x 12 1/4″ x 6″
  • Inside: 12″ x 12″ x 5 1/2″

USPS Priority Mail APO/FPO Large Flat Rate Box

The size of the package supports the ability to fill it with approved items with the same weight limit of 70 pounds. Shipping overseas can be a bit pricey, so this flat-rate shipping option makes it much easier to send items to loved ones.

What is USPS Regional Rate shipping?

USPS Regional Rate is a shipping method that provides a flat rate up to 15 or 20 pounds depending on which of the two box options you use. The difference between this and USPS Priority Mail Flat Rate is that it still takes distance traveled into account.

If you compare Regional Rate to both standard Priority Mail weight-based methods and other Priority Mail Flat Rate options, there are times when it will win out and when it will not. The savings can be substantial when it does win out, so it’s worth making the comparisons.

For example, if you compare Regional Rate Box A versus Medium & Large Flat Rate Boxes, Box A wins out for almost every Zone (up to 15 pounds, as that is the limit for Regional Rate Box A).

When comparing Regional Rate Box B versus Medium & Large Flat Rate Boxes up to 20 pounds, Regional Rate Box B wins from Zones 1-5, but 6-7 Medium Flat Rate wins, and for Zones 8-9 Large Flat Rate wins.

Much like USPS Priority Mail Medium Flat Rate Boxes, there are two different versions of Regional Rate Boxes for both A and B sizes.

USPS Priority Mail Regional Rate Box A1 and A2

USPS Priority Mail Regional Rate Box A

USPS Priority Mail Regional Rate A1 specs:

    Outside: 10 ⅛” (L) x 7 ⅛” (W) x 5” (H)
    Inside: 10” (L) x 7” (W) x 4 ¾” (H)

USPS Priority Mail Regional Rate A2 specs:

    Outside: 11 1/16” (L) x 2 ½” (W) x 13 1/16” (H)
    Inside: 12 15/16” (L) x 2 ⅜” (W) x 12 13/16” (H)

USPS Priority Mail Regional Rate Box B1 and B2

USPS Priority Mail Regional Rate Box B

USPS Priority Mail Regional Rate B1 specs:

  • Outside: 12 ¼” (L) x 10 ½” (W) x 5 ½” (H)
  • Inside: 12 ⅛” (L) x 10 ⅜” (W) x 5 ¼” (H)

USPS Priority Mail Regional Rate B2 specs:

  • Outside: 14 ½” (L) x 3” (W) x 16 ¼” (H)
  • Inside: 14 ⅜” (L) x 2 ⅞” (W) x 15 ⅞” (H)

When Should You Use Flat Rate Shipping?

As we’ve mentioned sporadically throughout, there’s no one-and-done choice when it comes to shipping your products. There are times when one method will be cheaper than another—sometimes substantially so. That’s why you need to make use of a platform like ShippingEasy that easily allows you to compare rates by carrier and service in one screen.

Do Other Carriers Offer Flat Rate Shipping?

They do, and we just so happened to write the only extensive guide around. Click below to download a free copy of The E-commerce Seller’s Guide to Flat-Rate Shipping, which features all services offered by USPS, UPS, and FedEx, breaks down the benefits of using which service in what instances, and takes a deeper dive into flat-rate shipping than we’ve even done in this article.

Get the Guide

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Rob Zaleski

Content Marketing Manager at ShippingEasy
I'm a digital content and social media nerd with a passion for telling stories, helping connect people, and finding joy in the details.