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All Things Shipping | February 13, 2021

When is USPS Priority Mail Flat Rate Cheaper than USPS Priority Mail?

The cost of shipping is the No. 1 challenge for three-quarters of online retailers, so it comes as no surprise that e-commerce businesses are often looking for ways to reduce their shipping expenses. Shipping software with discounted USPS rates can save you up to 89 percent on labels for service levels like USPS Priority Mail, USPS Priority Mail Flat Rate, and USPS First Class Package Service, etc.

As you begin to fulfill your orders, you’ll want to understand which of these service levels will offer the best rate for your packages. The most popular option for shipping domestic U.S. packages over one pound is USPS Priority Mail. It’s an affordable, timely service level for e-commerce retailers that includes tracking information. However, sometimes it is more cost-effective to use USPS Priority Mail Flat Rate service instead, depending on the item’s weight and distance traveled.

So, when should you choose USPS Priority Mail and when does it make sense to use USPS Priority Mail Flat Rate Boxes?

First, it’s helpful to get a quick overview of each. Let’s break it down:

USPS Priority Mail Flat Rate

The USPS has free Flat Rate boxes that you can order online or pick up at your local Post Office. These packages typically ship within 1-3 business days for a flat fee—regardless of how far they’re traveling—and include tracking information. Smaller Flat Rate Boxes often cost less than larger ones, so you’ll usually want to get the smallest option that will fit your product(s).

When you use USPS Priority Mail Flat Rate shipping, it’s easier to predict shipping costs: you know exactly what you’re getting without calculating shipping zones and measuring dimensions and weights.

The Bamboo Tranquility team sticks to Flat Rate shipping, “Since I ship bed sheets, they’re heavy enough to make the Flat Rate worth it, and they fit perfectly,” says founder Milovan Crowley.

Here’s how the 2021 USPS Priority Mail Flat Rate pricing breaks down if you’re using Shippo’s shipping software.

2021 USPS Priority Mail Flat Rate Boxes
Products Dimensions Shippo Rate
Flat Rate Envelope 12 ½” x 9 ½” $7.40
Legal Flat Rate Envelope 15” x 9 ½” $7.70
Padded Flat Rate Envelope 12 ½” x 9 ½” $8.00
Small Flat Rate Box Outside: 8 11/16” x 5 7/16” x 1 ¾”
Inside: 8 ⅝” x 5 ⅜” x 1 ⅝”
$7.90
Medium Flat Rate Box – 1 (Top Loading) Outside: 11 ¼” x 8 ¾” x 6”
Inside: 11” x 8 ½” x 5 ½”
$13.75
Medium Flat Rate Box – 2 (side-loading) Outside: 14” x 12” x 3 ½”
Inside: 13 ⅝” x 11 ⅞” x 3 ⅜”
$13.75
Large Flat Rate Boxes Outside: 12 ¼” x 12 ¼” x 6”
Inside: 12” x 12” x 5 ½”
$19.30

We’ve also put together a handy, in-depth guide to USPS Flat Rate box sizes and pricing where you can find out more about this shipping option.

USPS Priority Mail

USPS Priority Mail uses the weight of your package and the distance it travels to determine how much you’ll have to pay to get it in your customer’s hands.

Like the Flat Rate alternative, USPS Priority Mail arrives in 1-3 days. It includes tracking information, as well, which is helpful both to your operations team and to the customer.

E-commerce businesses have more flexibility when it comes to packaging with Priority Mail. Rather than being required to use USPS-branded packaging, you can use your own custom boxes and envelopes. Believe it or not, this matters: 40 percent of consumers would be more likely to purchase from a brand again if it arrived in premium packaging, according to Dotcom Distribution’s “Driving Customer Loyalty With Fast Delivery and Quality Packaging” survey.

USPS Priority Mail vs. USPS Priority Mail Flat Rate

A good rule of thumb is to ship small, heavy packages via Flat Rate, and large, lightweight items via Priority Mail. But there’s more to consider than just that.

In addition to size and weight, you’ll want to think about where the package is going. Items traveling far distances will cost more with Priority Mail, but the cost is unchanged if you’re going the Flat Rate route.

This leads us to the ultimate question: what is the threshold that, when passed, each USPS Priority Mail Flat Rate product becomes more affordable than the USPS Priority Mail alternative?

This threshold varies based on the Flat Rate product, weight, and distance traveled (also known as Zone). To help you understand when is the right time to switch, we compared the 2021 USPS postage rates for Priority Mail and Priority Mail Flat Rate in the charts below.

When Should I Use a Small Flat Rate Box?

Let’s say you have an item that would fit snugly in a Small Flat Rate Box (measuring 8 ⅝ x 5 ⅜ x 1 ⅝ inches). When does it make sense to ship this item in your own packaging via USPS Priority Mail and when should you use the Small Flat Rate Box and service level?

Well, that depends on how much that item weighs and where it’s traveling. With that information, you can determine which option is more affordable. Here’s the full chart comparing the two service levels up to 20 pounds (which is the weight limit for Flat Rate products).

Blue cells reflect that USPS Flat Rate is the more cost-effective option. Green cells indicate you’re better off with USPS Priority Mail. Please keep in mind that these are prices with Shippo’s discounts.

Use Priority Mail Use Priority Mail Flat Rate
USPS Small Flat Rate Box (8 11/16” x 5 7/16” x 1 ¾”) – $7.90
Weight (pounds) Zones 1-2 Zone 3 Zone 4 Zone 5 Zone 6 Zone 7 Zone 8 Zone 9
0.5 $7.16 $7.46 $7.67 $7.88 $7.90 $7.90 $7.90 $7.90
1 $7.16 $7.46 $7.67 $7.88 $7.90 $7.90 $7.90 $7.90
2 $7.32 $7.59 $7.81 $7.90 $7.90 $7.90 $7.90 $7.90
3 $7.32 $7.59 $7.81 $7.90 $7.90 $7.90 $7.90 $7.90
4 $7.32 $7.59 $7.81 $7.90 $7.90 $7.90 $7.90 $7.90
5 $7.32 $7.59 $7.81 $7.90 $7.90 $7.90 $7.90 $7.90
6 $7.32 $7.59 $7.81 $7.90 $7.90 $7.90 $7.90 $7.90
7 $7.32 $7.59 $7.81 $7.90 $7.90 $7.90 $7.90 $7.90
8-20 $7.32 $7.59 $7.81 $7.90 $7.90 $7.90 $7.90 $7.90
21-70 $7.90 $7.90 $7.90 $7.90 $7.90 $7.90 $7.90 $7.90

When Should I Use a Medium Flat Rate Box?

The USPS provides two Medium Flat Rate Boxes. Here are the inside dimensions for each:

  • 11 x 8 ½ x 5 ½ inches
  • 13 ⅝ x 11 ⅞ x 3 ⅜ inches

If your item fits and falls into one of the following blue squares, then ship it with a Medium Flat Rate Box.

USPS Medium Flat Rate Box (11 ¼” x 8 ¾” x 6”) – $13.75
Weight (pounds) Zone 0,1,2 Zone 3 Zone 4 Zone 5 Zone 6 Zone 7 Zone 8 Zone 9
0.5 $7.16 $7.46 $7.67 $7.88 $8.34 $8.52 $8.80 $13.75
1 $7.16 $7.46 $7.67 $7.88 $8.34 $8.52 $8.80 $13.75
2 $7.79 $7.96 $8.24 $8.85 $10.44 $11.01 $11.69 $13.75
3 $8.00 $8.35 $8.72 $9.64 $12.70 $13.75 $13.75 $13.75
4 $8.10 $8.58 $9.21 $10.43 $13.58 $13.75 $13.75 $13.75
5 $8.20 $8.63 $9.53 $10.60 $13.58 $13.75 $13.75 $13.75
6 $8.31 $8.67 $9.64 $10.60 $13.58 $13.75 $13.75 $13.75
7 $8.73 $9.27 $9.66 $10.60 $13.58 $13.75 $13.75 $13.75
8 $8.79 $9.27 $9.66 $10.60 $13.58 $13.75 $13.75 $13.75
9 $8.93 $9.27 $9.66 $10.60 $13.58 $13.75 $13.75 $13.75
10 $8.93 $9.27 $9.66 $10.60 $13.58 $13.75 $13.75 $13.75
11 $8.93 $9.27 $9.66 $10.60 $13.58 $13.75 $13.75 $13.75
12-20 $8.93 $9.27 $9.66 $10.60 $13.58 $13.75 $13.75 $13.75
21-70 $13.75 $13.75 $13.75 $13.75 $13.75 $13.75 $13.75 $13.75

When Should I Use a Large Flat Rate Box?

A Large Flat Rate Box fits items measuring the following dimensions: 12 x 12 x 5 ½ inches. If your product falls into a blue cell below, then consider shipping with a Large Flat Rate Box.

USPS Large Flat Rate Box* (12 ¼” x 12 ¼” x 6”) – $19.30
Weight (pounds) Zones 1-2 Zone 3 Zone 4 Zone 5 Zone 6 Zone 7 Zone 8 Zone 9
0.5 $7.16 $7.46 $7.67 $7.88 $8.34 $8.52 $8.80 $14.25
1 $7.16 $7.46 $7.67 $7.88 $8.34 $8.52 $8.80 $14.25
2 $7.79 $7.96 $8.24 $8.85 $10.44 $11.01 $11.69 $19.30
3 $8.00 $8.35 $8.72 $9.64 $12.70 $14.10 $16.45 $19.30
4 $8.10 $8.58 $9.21 $10.43 $14.80 $16.78 $18.96 $19.30
5 $8.20 $8.63 $9.53 $11.50 $16.02 $18.08 $19.30 $19.30
6 $8.31 $8.67 $9.64 $11.50 $16.02 $18.08 $19.30 $19.30
7 $8.73 $9.58 $10.13 $11.50 $16.02 $18.08 $19.30 $19.30
8 $8.79 $9.58 $10.22 $11.50 $16.02 $18.08 $19.30 $19.30
9 $9.08 $9.58 $10.22 $11.50 $16.02 $18.08 $19.30 $19.30
10 $9.08 $9.58 $10.22 $11.50 $16.02 $18.08 $19.30 $19.30
11 $9.08 $9.58 $10.22 $11.50 $16.02 $18.08 $19.30 $19.30
12-20 $9.08 $9.58 $10.22 $11.50 $16.02 $18.08 $19.30 $19.30
21-70 $19.30 $19.30 $19.30 $19.30 $19.30 $19.30 $19.30 $19.30

Moving Forward With Your E-commerce Shipping Needs

When it comes to choosing between USPS Priority Mail Flat Rate vs. USPS Priority Mail, there is no one-size-fits-all answer. It really comes down to the product you’re shipping, how far it’s going, and how much you want to spend.

My Coffee Stix, for example, uses a combination of USPS shipping options. For orders that contain one or two bags of coffee, they use the USPS Priority Padded Flat Rate Envelope. But for larger orders, they turn to other options. “Our shipping cost ranges from $7.16-$11.50,” says co-founder Felice Lacroix. “They’re not only economical but also very fast; our customers love how fast they get their coffee.”

It’s important to have a shipping software that’s flexible enough to provide you the shipping options you need to run a cost-effective e-commerce business. Shippo saves you up to 40 percent on USPS labels, and helps you streamline your fulfillment process. What’s more, Shippo offers users a free Starter account. Users simply pay the cost of postage, plus $0.05 per label (this fee is waived when using Shippo’s carrier accounts). For some real-time examples of how much you can save with Shippo, visit our USPS shipping calculator

Alexandra Sheehan is a copywriter who works with B2B companies in the retail, e-commerce, and travel sectors to create strategies and expert long-form, website, and blog content. Learn more at thealexsheehan.com.

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