Flat Rate vs. Cubic Shipping: How To Get The Best Value For Each Package

USPS Priority Mail Flat rate gives merchants affordable options for multiple package sizes

  • Written by Meg Thunell
  • Published on November 3, 2021
  • Time to read 9 minutes

If you own an e-commerce business, you’ve probably had someone tell you Priority Mail flat-rate shipping is the only way.

You’ve also probably had people tell you flat rate shipping is a scam.

Here’s the honest truth straight from our shipping experts: there is not a universally “right” way to ship a package. The right solution will vary by box size, package weight, and final destination.

What is flat-rate shipping?

Most people who talk about flat rate shipping are talking about Priority Mail Flat Rate, but that is only one type of USPS flat rate shipping USPS (the other two are Priority Mail Regional Rate and Priority Mail Cubic).

Each of these “flat” rate services uses the same network to provide the same services (tracking, insurance, guaranteed delivery window, etc.), but the pricing structure varies based on package weight, size, and destination.

Priority Mail Flat Rate—designed to simplify shipping large, heavy packages that need to travel long distances, Priority Mail Flat Rate charges a flat fee for any package under 70 pounds that can fit into a free flat rate box.

Priority Mail Regional Rate—similar to Priority Mail Flat Rate, Priority Mail Regional Rate is perfect for medium-sized packages between 15–20 pounds that don’t have to travel too far to reach their final destination.

Priority Mail Cubic Rate—best for small, heavy packages, Priority Mail Cubic charges a flat shipping rate, calculated by delivery zone, for packages that weigh less than 20 pounds and have dimensions smaller than 18” for the longest side (basically the size of a shoebox).

Priority Mail Flat RatePriority Mail Regional RatePriority Mail Cubic Rate
Package SizeWhatever can fits into a flat rate boxMedium-sized but still what fits into a flat rate boxSmall packages no bigger than .5 cubic feet
Package WeightUp to 70 pounds15–20 poundsUp to 20 pounds
Package DestinationLong distancesShort-range shippingRates change by zone

For the purposes of this article, we are going to lump Priority Mail Flat Rate and Priority Mail Regional Rate together and compare them against USPS Priority Mail Cubic.

USPS Priority Mail flat rate vs. USPS Priority Mail Cubic

One of the most significant advantages of USPS flat-rate shipping is that shippers don’t need to weigh their packages or buy their own boxes. For e-commerce companies that self-fulfill orders, those two benefits alone deliver much-needed time savings.

USPS Priority Mail Flat rate also has three boxes ranging from small to large, which gives merchants affordable options for multiple package sizes. The standard pricing model of Priority Mail flat rate allows e-commerce businesses to estimate their monthly or quarterly shipping costs accurately.

The problem is that even if your shipping estimates are accurate, if you aren’t careful, Priority Mail flat rate will cost you more than you need to pay. If you aren’t filling that flat rate box to the brim, or if you’re not shipping something all the way across the country, regional or cubic rates could save you anywhere from $1–$4 a package.

Depending on where you’re shipping, Priority Cubic can save you anywhere from $7–$123 per package

Which means those free boxes aren’t really free at all.

USPS Priority Cubic doesn’t provide free boxes, but it does deliver significantly better rates than Priority Mail flat rate for small, heavy packages.

Priority Cubic is a hidden secret in the shipping world, partially because this program is only available to companies that ship over 50,000 packages a year (small and mid-sized e-commerce companies can get around this requirement by using a shipping aggregator like eHub).

Depending on where you’re shipping, Priority Cubic can save you anywhere from $7–$123 per package over Priority Mail flat rate prices for small, heavy items. With savings like that, it doesn’t matter that Priority Cubic doesn’t provide free boxes. You can design and produce fantastic packaging branded with your company logo and still save money on shipping.

So, how do I know which program to use when?

That’s the magic question, isn’t it?

Most business owners don’t have the time or energy to figure out the most cost-effective shipping option for every single package they send—that’s why Priority Mail flat rate is so popular in the first place!

Shipping is EHub’s business, but it’s also our passion. Our shipping API fully integrates with over 200 e-commerce and shipping websites and gives you side-by-side rate comparisons, which makes finding the right shipping solution for every shipment fast and easy.

Even better? Our status as a significant shipping aggregator means you’ll have access to pre-negotiated discount rates lower than you could get working directly with carriers. If you’re curious about what Ehub can do for you, let’s talk.

Flat rate FAQs

Still have questions about Priority Mail flat rate and Priority Cubic? Check out our FAQ below.

How much are USPS Priority Mail flat-rate shipping boxes?

Zero. Zilch. Zip. Nada. Nothing. But remember, using free boxes doesn’t necessarily mean you are saving money.

How long does USPS Priority Mail flat rate shipping take?

While it has no money-back guarantee, all three USPS Priority shipping options promise delivery within 1–3 business days.

How much is flat-rate shipping?

Flat Rate Envelopes$7.75
Legal Flat Rate Envelopes$8.05
Padded Flat Rate Envelopes$8.45
Small Flat Rate Box$8.25
Medium Flat Rate Box$14.25
Large Flat Rate Box$19.20
APO/FPO/DPO Large Flat Rate Box$17.70

Does USPS Priority Mail flat rate ship to Hawaii, Alaska, Puerto Rico, Canada, and Mexico?

Hawaii, Alaska, and Puerto Rico? Yes. (Side note: Guam, American Samoa, and the US Virgin Islands qualify too!)

Canada and Mexico? No.

Even though Canada and Mexico are closer to the mainland US than Alaska or Guam, Priority Mail flat rates aren’t based on proximity alone. Packages destined for Canada and Mexico must cross international borders, so they are subject to customs agreements and additional taxes/inspections.

The good news is that there is a Priority Mail International program that delivers packages to our North American neighbors in 6–10 business days.

Can I use my branded boxes for USPS Priority Mail flat rate?

Both USPS Priority Mail flat rate and Regional flat rate require the use of USPS Priority Mail boxes. If you want to use your own packaging or boxes marked with your company brand, you can still pay similar rates using USPS Priority Mail Cubic.

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