Shipping Hack: How USPS Priority Mail Cubic Softpack Can Save You Big

If you’re running an online store, you already know that shipping costs can eat into your profit margins fast. But here’s the good news: there’s a USPS shipping method that not a lot of sellers talk about... but it can help you save significantly per package, especially when you're sending heavier items.

It’s called Priority Mail Cubic Softpack, and it’s one of the best-kept secrets for small businesses trying to keep costs down without compromising on shipping speed.

What Is USPS Cubic Softpack?

USPS Priority Mail Cubic pricing is a special discounted rate for small, heavy packages that ship fast—typically in 1–3 days.

But here’s the twist: unlike traditional Priority Mail which uses dimensional weight (length × width × height), Cubic Softpack only considers the length and width of your outer mailer. That means you can ship dense or boxed items inside a soft mailer without paying more for their actual bulk.

But Why Put a Box Inside a Poly Mailer?

Normally, USPS Priority Mail uses dimensional weight- factoring in length, width, and height. That means bulkier packages cost more, even if they’re not heavy. But Cubic Softpack only measures length and width of your soft outer mailer.

If your boxed item fits snugly into a poly mailer, USPS considers it a soft pack, not a box. This gets you lower rates without compromising on protection.

How It Works: The "Box-in-a-Bag" Method

This shipping hack is officially allowed by USPS and works like this:

Place your boxed item into a poly mailer, padded envelope, or soft pack.
Seal the mailer normally (but do NOT tape the sides rigidly).
Measure the mailer while empty — just the length and width.
Round down to the nearest ¼ inch (for example, 13.7” becomes 13.5”).

📦 Because the package remains soft and flexible, USPS categorises it as a soft pack, not a box—meaning you get the lower Cubic pricing tier.

Real Savings Add Up

Here’s a real-world example to show how much you could save:

Shipping a 10 lb item to Zone 8	Cost
Regular Priority Mail (Box)	$34.65
Cubic Softpack Rate	$9.87
Savings Per Package	$24.78

Multiply that by 100 packages a month, and you're looking at $2,478 in monthly savings, just from switching to the Cubic Softpack method.

How to Use It on Pirate Ship and Other Platforms

We’ve been using this method ourselves with great success. If you’re purchasing shipping labels online, most platforms will have a version of this shipping method.

On Pirate Ship (our go-to tool for labels), you’ll want to choose:

“Envelope, Padded Envelope, Poly Bag, Soft Pack, or Box in a Bag”

This ensures your label is priced correctly using the Cubic Softpack rate.

📝 Tip: You may need to measure and input your mailer size manually the first time.

Important Guidelines to Remember

To make sure your package qualifies for the Cubic Softpack rate:

Do NOT tape down the sides of the mailer – it must remain soft and flexible.
Always measure the mailer while it’s empty (length and width only).
Round down to the nearest quarter inch.
Maximum weight is 20 lbs.
Your item must fit securely inside the mailer without damaging or distorting it.

What Kind of Products Work Best?

Cubic Softpack is ideal for:

Clothing or apparel (especially in boxes or folded neatly)
Cosmetics or skincare kits
Books or journals
Gift boxes
Electronics (like chargers, gadgets, or accessories)

Basically, if it fits in a box and that box fits in a soft mailer: you’re golden.

Bonus: Our Mailers Are Built for This

If you want to try this out, our durable, protective mailers are specifically designed to work with the Cubic Softpack method. They're flexible enough to stay within USPS guidelines while still keeping your products safe and presentable.