USPS Flat Rate Boxes, Envelopes, & Pricing

What are USPS Flat Rate Boxes & Envelopes?

USPS Flat Rate Boxes & Envelopes are just that - boxes and envelopes with flat postage rates, no matter how heavy the package is. If you're shipping heavy, small items, Flat Rate Boxes & Envelopes are the best bang-for-your-buck option. Take advantage of USPS flat rates by keeping a supply of these around your fulfillment operations. USPS provides these flat rate boxes and envelopes for free at most Post Offices or will ship them to you at the USPS Storeopens in new tabopen_in_new. Using a USPS Flat Rate Box or Envelope can cut your shipping costs by 50%. This is a great way to prepare for peak shipping.

In addition to large Flat Rate Boxes, small Flat Rate Boxes and medium Flat Rate Boxes, prices are supported through the EasyPost API by using our predefined packages. So it's easy to set up internally and start collecting savings from your Flat Rate Boxes & Envelopes as soon as you integrate with EasyPost.

What is the Postage Cost For USPS Flat Rate Boxes & Envelopes?

We've broken the pricing into the table below. Note: The rates below reflect USPS Commercial Pricing, which is available when shipping via EasyPost. If you're interested in signing up for USPS Commercial Pricing, contact us and we can set you up.

How You Can Use USPS Flat Rate Boxes and Envelopes to Save on Shipping

One of the many benefits of shipping with USPS Flat Rate Boxes is that the boxes themselves are free. You can pick them up at any post office or USPS will actually ship the boxesopens in new tabopen_in_new to you for free. In order to have access to the USPS Priority Mail Flat Rate pricing, you must use one of USPS designated Flat Rate Boxes; you cannot use just any box of the same dimensions. This can be a drawback for some because, unless you are a high volume shipper, you cannot get customized boxes with specific branding. If using non-customized boxes is not a deal breaker for you, USPS Flat Rate Shipping is a great tool to spend less on shipping.

With a USPS Flat Rate Box or Envelope, you will be able to ship any item of any weight up to 70lbs to any location for the same price assuming it fits properly, without any bulging, within its designated box or envelope. That being said, let's dive into the different USPS Priority Mail Box and Envelope options.

USPS Priority Mail Flat Rate Envelopes

There are 5 different options for USPS Flat Rate Envelopes. Each envelope is unique in size and structure and has differing use cases.

Flat Rate Envelope

12 ½" x 9 ½"

Flat Rate Envelopes are great for documents, manuscripts, and applications.

Window Envelope

10" x 5" or 12 ½" x 9 ½"

Window Envelopes are great for pre-addressed documents.

Small Flat Rate Envelope

10" x 6"

Small Flat Rate Envelopes are great for greeting cards, photos, small documents, and gift cards.

Padded Flat Rate Envelope

12 ½" x 9 ½"

Padded Flat Rate Envelopes are great for books, small clothing, and jewelry.

Legal Flat Rate Envelope

15" x 9 ½"

Legal Flat Rate Envelopes are designed specifically for important legal documents.

USPS Flat Rate Boxes

When shipping with USPS Flat Rate Boxes there are a few different sizes to consider.

USPS Priority Mail Flat Rate Box: SMALL

Small USPS Priority Mail Flat Rate Boxes are great for items that are small, but dense or rigid like larger pieces of clothing, brochures, and small electronics.

Outside: 8 ¹¹⁄₁₆" x 5 ⁷⁄₁₆" x 1 ¾"
Inside: 8 ⁵⁄₈" x 5 ³⁄₈" x 1 ⁵⁄₈"

USPS Priority Mail Flat Rate Box: MEDIUM

Medium Priority Mail Flat Rate Boxes come in 2 different options: top-loading and side-loading.

Top-loading

Outside: 11 ¼" x 8 ¾" x 6"
Inside: 11" x 8 ½" x 5 ½"

Side-loading

Outside: 14" x 12" x 3 ½"
Inside: 13" x ⁵⁄₈" x 11 ⁷⁄₈" x 3 ³⁄₈"

As you can see while these 2 boxes are both considered to be "medium sized" they are vastly different in their dimensions and are designed to fit different types of items.

USPS Priority Mail Flat Rate Box: LARGE

Large USPS Priority Mail Flat Rate Boxes are great for larger items such as laptops, computers, gifts, and larger electronics.

Outside: 12" x 12 ¼" x 6"
Inside: 12" x 12" x 5 ½"

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

APO/FPO/DPO boxes are specifically designed for sending to troops overseas. These Priority Mail boxes are the same exact dimensions as the USPS Flat Rate Large Box, but have been altered slightly on the outside to include a message of support for military personnel. These boxes also have a discounted flat rate.

Outside: 12" x 12 ¼" x 6"
Inside: 12" x 12" x 5 ½"