Understanding USPS Flat Rate Shipping: Shipping Flat Rate for Ecommerce


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Understanding USPS Flat Rate Shipping: Shipping Flat Rate for Ecommerce

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Shipping margins are critical to the profitability and sustainability of ecommerce businesses. If you aren’t shipping your products in the most affordable and efficient way, then you’re leaving money on the table. One possible way to reduce shipping costs is with USPS flat rate shipping.

With flat rate shipping, the weight of a package does not affect the cost of shipping. If you ship heavy items, then this will save you money. On the flip side, flat rate shipping can be more expensive if your packages are lightweight.

How do you decide if flat rate shipping is right for your ecommerce business?

This article provides a detailed breakdown of flat rate shipping, how it works, and whether or not you should start using it.

What is flat rate shipping?

USPS Priority Flat rate is a shipping service provided by the US Postal Service that offers 1-3 day shipping services for a single flat rate. Flat rate service is attractive to many shippers since it essentially eliminates the need to weigh your packages in order to calculate shipping costs. As long as your package weights under 70 pounds, it can ship with USPS Priority Flat Rate service.

Why shipping flat rate is perfect for ecommerce shops

Flat rate shipping is an appealing shipping option for a few reasons based on how shipping costs are typically calculated.

First, the farther the distance or shipping zones you ship your orders to, the more expensive it typically is, and this only increases the heavier the order is.

Additionally, dimensional weight is a commonly used pricing technique that is calculated by multiplying length times width times height of a package. Freight carriers like USPS, Fedex, and UPS calculate shipping charges based on whichever number is greater: the actual weight of the package or its calculated dimensional weight.

Flat rate shipping simplifies things quite a bit and can be a great option for certain brands, especially those who are smaller and self-fulfill orders.

Note: Flat rate shipping can also refer to a technique for deciding how to price your shipping, when you charge a fixed shipping rate for every order (rather than offering free shipping or passing the real-time shipping rate on to the customer). For the purpose of this article, we are talking about the USPS flat rate shipping service.

How does flat rate shipping work? 3 steps

Flat rate shipping means the price of shipping is not connected to the weight, shape, or size of the shipped item, hence the term “flat rate.” It’s based on how much you can stuff into the USPS-issued box. Ecommerce businesses ship products at a flat rate by following these steps.

1. Get a USPS flat rate box

Instead of packing items in your own box, choose from a variety of flat rate boxes at your local USPS office. There are small, medium, and large flat rate boxes; And the bigger the box, the higher the shipping prices. The maximum allowed weight is 70 pounds.

2. Pack your items and address your box

Pack your items in the USPS flat rate box and address the package.

3. Ship your items

Bring your flat rate box to a USPS office and ship the box using Priority Mail.

How much is USPS flat rate shipping?

The cost of flat rate shipping in the United States depends on the size of the box. See the following chart for detailed pricing and box sizes.

Flat rate options & Flat rate pricing:

Flat rate boxCommercial base pricePrice at post officeDimensions
Flat Rate Envelope$6.95$7.3512 1/2″ x 9 1/2″
Small Flat Rate Box$7.50 $7.90 8 11/16″ x 5 7/16″ x 1 3/4″
Medium Flat Rate Box – 1 (top loading)$12.80$14.35 11 1/4″ x 8 3/4″ x 6″
Medium Flat Rate Box – 2 (side-loading)$12.80$14.3514″ x 12″ x 3 1/2″
Large Flat Rate Box$17.60$19.9512 1/4″ x 12 1/4″ x 6″

How long is the flat rate shipping time?

Packages shipped with flat rate shipping are typically delivered within one and three days (not counting Sundays). They are shipped with via Priority Mail and the exact time of delivery depends on where the package is sent from and its final destination.

Why use flat rate shipping for ecommerce?

Flat rate shipping can generate huge savings when it comes to small business shipping by having a lower, predictable cost. It won’t make sense for every ecommerce business, but is used most often by small business owners that pack boxes themselves and run to the post office to ship orders.

1. Package anything up to 70 pounds

There is no need to worry about weighing and calculating shipping costs for each package. As long as your package is under the 70 lb. limit, then you can ship it with flat rate shipping.

2. Automatically insured up to $50

Instead of having to purchase shipping insurance separately, it’s already taken care of to an extent. Every flat rate package is automatically insured up to $50. If the contents are worth $50 or less, your costs will be covered if lost. They also can include a certificate of mailing for even more peace of mind!

3. Drop off at any USPS store

When your package is ready to go, you can drop it off at any USPS office to be shipped.

4. USPS tracking included

Another ecommerce shipping essential, simply enter the package number into USPS’s tracker and you can follow its progress all the way to delivery.

USPS Priority Mail restrictions

You don’t have to worry about weight with flat rate shipping, but that doesn’t mean you can send absolutely anything. The following items cannot be shipped through USPS Priority Mail:

  • Explosives
  • Firearm ammunition
  • Air bags
  • Alcohol
  • Gasoline
  • And more

How to outsource your ecommerce shipping with ShipBob

If your ecommerce business is outgrowing your current shipping methods, another way you can bring down your shipping costs and improve margins is with optimized shipping. ShipBob is a third-party logistics (3PL) provider that streamlines your shipping using integrated technology and multiple fulfillment centers. Here’s how fulfillment with ShipBob works.

1. Connect your store

ShipBob’s platform syncs with your ecommerce store (to handle, for example, Shopify shipping and WooCommerce shipping), so you can easily manage inventory and monitor the fulfillment process from start to finish.

2. Store your inventory at ShipBob’s fulfillment centers

Store your products across ShipBob’s national network of fulfillment centers. This puts your products closer to your customers for reduced shipping costs and transit times.

3. Have orders fulfilled automatically

Orders placed at your store are pushed automatically to the optimal ecommerce fulfillment center, then picked, packed, and shipped by ShipBob’s team.

4. Customers get tracking info

Ecommerce order tracking information is sent to the customer once the package has been shipped, so they can follow the progress of their package.


Flat rate shipping is a popular USPS mail type that can be a valuable tool for reducing shipping costs, but only if your packages are heavy enough, so that the flat rate is cheaper than the regular rate. You can find out if flat rate shipping is worthwhile for your ecommerce business by comparing the flat rate to the rate that you’re currently paying to ship your products. If the majority of your products are heavier but under 70 pounds and your current rates are higher than the flat rate, then flat rate shipping can save you money.

Learn more about outsourcing fulfillment

Download the free e-guide, “How to Choose a 3PL for Your Ecommerce Business” and learn more about how a 3PL can help your company scale and get tips for choosing a fulfillment partner.

How does flat rate shipping work?

To use USPS flat rate shipping, you need to get a USPS flat rate box from your local USPS office (small, medium, or large size), add no more than 70 pounds of contents that fit inside the box, label an address, and ship the box at a USPS office using Priority Mail.

Is flat rate shipping cheaper?

USPS flat rate shipping can be cheaper than other shipping methods because you can optimize what you fit in the flat rate box, but the major benefit is that it makes it easier to budget for shipping costs because you can predict the cost every time.

How long does it take to get flat rate shipping?

Flat rate shipping via USPS Priority Mail typically takes 2-3 business days to be delivered during non-peak seasons.

What is free flat rate shipping?

Free flat rate shipping is when you don’t charge ecommerce customers for shipments that go out using this USPS service. Free shipping can be a great way to get more shoppers to complete their purchase online, and it’s often most cost-effective when you offer it in exchange for a minimum spend threshold. This helps preserve your margins while increasing your average order value.

Written By:

Kristina is the Director of Marketing Communications at ShipBob, where she writes various articles, case studies, and other resources to help ecommerce brands grow their business.

Read all posts written by Kristina Lopienski