Box in a Bag Shipping: What You Need to Know

Putting your box in a bag is a "cheat code" that sometimes lets you save even more money when shipping USPS Priority Mail Cubic
Box in a Bag
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If you’ve spent any time on our site (or on online shipping software in general), you likely know that shipping USPS Priority Mail Cubic is the best way to save money when shipping packages. However, what if we told you there was a way in which you could save even more? Well, by placing your box inside a poly mailer or envelope, you might be able to! This is a special trick known as the “box in a bag” shipping method. Lots of eCommerce shippers use this method every day to cut down their shipping costs, because when you’re shipping tons of packages, every cent counts!

What Do You Mean, Box in a Bag?

The “box in a bag” shipping method is pretty much exactly how it sounds. All you need to do is place your box inside a poly mailer, or a shipping bag. Seal the bag, buy your postage, send it off, and call me Captain Obvious!

Keep in mind that you’ll only be able to take advantage of this “cheat code” when you use online shipping software to buy discounted USPS postage online.

How Shipping Your Box in a Bag Can Save You Even More Money

We wish we had a better explanation here than “That’s just the way it is”…but this phenomenon of saving money by shipping your box in a bag is just how the math works out sometimes. Hey, we don’t make the rules! We just know how to tip the scales in our favor.

Believe it or not, shipping a box in a bag applies beyond just Priority Mail Cubic. In fact, a lot of shippers utilize the box in a bag method when sending lightweight items in smaller boxes via USPS First Class Package Service. Some examples of these kinds of parcels include shipping perfumes and fragrances.

What Cubic Pricing Tier Does My Box in a Bag Fall Into?

To calculate your cubic pricing tier, simply measure the Length and Width in inches before you put your box inside of the bag. That last part is important! USPS technically only cares about the dimensions of the empty envelope, so don’t worry about putting box inside of your bag when measuring your dimensions. If either dimension exceeds the nearest quarter inch, round down to the nearest 1/4 inch (for example, 7.8″ inches turns into 7.75″ inches).

Once you’ve got the length and width, add those together to calculate your cubic pricing tier:

  • 0.1 – Envelopes with a Length plus Width measuring more than 0″ up to 21″
  • 0.2 – Envelopes with a Length plus Width measuring more than 21″ up to 27″
  • 0.3 – Envelopes with a Length plus Width measuring more than 27″ up to 31
  • 0.4 – Envelopes with a Length plus Width measuring more than 31″ up to 34″
  • 0.5 – Envelopes with a Length plus Width measuring more than 34″ up to 36″

Once you’ve got the pricing tier that your bag belongs to, check the table below to see how much it’ll cost to ship!

Pay Attention to the Restrictions

As always, you need to be aware of certain restrictions when it comes to shipping your box in a bag. We’ve listed those out for you below.

First and foremost, neither your bag’s length or width can exceed 18″ in either direction. Also, just like regular Priority Mail Cubic, your bag’s total weight cannot exceed 20 pounds (lbs).

Last but certainly not least, you need to make sure you’re using the right kind of envelope or poly mailer. USPS will not allow you to ship the “box in a bag” method with expandable envelopes. You know the type—they’re the ones with the crease in the middle that compresses down and up to create more space inside the bag. Here’s a picture, to show you the kind of envelope we’re talking about:

Expandable Envelope

These kinds of envelopes are a no-go! If you’re using an expanding envelope like the one pictured above, USPS will treat it as a 3-dimensional box. As a result, they will charge you accordingly via their Automated Package Verification System (APV)…and no one wants that to happen.

Can I Use Any Kind of Bag? A Trash Bag Perhaps?

A lot of people wonder if they can use a trash bag to ship their box in a bag. The truth is, there’s a little bit of grey area here. USPS technically won’t restrict you from using a trash bag, but here’s what it comes down to: whatever kind of bag you use, you need to make sure it doesn’t “form fit” to the box. By “form fit,” we mean don’t tape the bag too tightly around the box. If the bag form fits to the box, then it will make the package look like a box. If it looks like a box and is shaped like a box, USPS will charge you for shipping a box. You catch our drift?

Have any questions about saving money by shipping your box in a bag? Leave a comment below!


Post a Comment
  1. Nancy

    So, can you use a brown grocery bag, put the box in, then just seal the end of the paper bag?

    • Rockwell Sands

      Technically, yes! Just as long as it doesn’t clearly look like a box inside of the bag.

    • Gladys Marie Snyder

      No you can’t use a grocery bag because it is gusseted (expandable)….it has to be a flat bag, not one you fold out to make bigger.

  2. Mary Prola

    So for those of you that think it is ok to use a FLAT RATE USPS box inside a bag to save $ it is not. It is a violation of USPS regulations-so be aware

  3. Dan

    Can you use a USPS priority mail box as the inner box since it’s still going USPS Priority Mail? If so is a flat-rate box also acceptable?

    • Rockwell Sands

      Hi Dan, as long as the box is concealed by the bag outside of it, you can use any box you like! Just make sure it doesn’t look like a box, and that the bag doesn’t “form fit” to the outside of it.

  4. Travis

    Is there a spot in eBay like there is in pirate ship to select bag instead of box?

    • Rockwell Sands

      I’m not sure Travis! I believe you can pre-select your packaging and choose envelopes or mailers instead of boxes…that would seem like it makes the most sense 🙂


      Travis, you don’t have to ship thru ebay… even though it’s easy… just use pirate ship 🙂

  5. Sammy

    Do you measure the length with the top flap open or closed Thanks

  6. Ahlora

    Thank you for clarifying this!

  7. Yogi

    Hi. I am confused. I thought I had the rules figured out until the last part where you talk about trash bags. Where you speak about the trash bag at the end, are you hinting that if you do have a bag or trash bag that is longer than 18 inches in any dimensions, that that is acceptable?

    I want to use cubic shipping, box in a bag. I have an item in a box that weights 10 pounds and has dimensions of 18x10x6. Is it ok/”legal” to put that boxed item into a poly mailer that is say 36 x 19 (or some dimension that will allow you to fit an 18″ long box inside a bag and fold it over to fit the overall length)? And maybe add stuffing to the sides so that it is not shaped like a box? I’m not sure what to do.

  8. Michelle H

    I’ve been told you can wrap your item in bubble wrap & use this method too. If that’s the case, you really don’t have length & width of an envelope. How do you note measurements?

    • Rockwell Sands

      Bubble wrap would be fine Michelle, you’ll just need to make sure it’s not form fit to the box! So you’ll want to give yourself plenty of extra room before cutting the bubble wrap itself. Before you put the box inside of it, try to fold it over so it looks like an envelope, and then measure the length and width before you place the box inside of it. Hope this helps!

  9. Lauren

    This is awesome! I was having trouble understanding the bag in a box method, and this answered all my questions! I’m excited to save even more on shipping.

    • Bianca

      I know people who ship box in bags and they form fit the boxes. They also buy the shipping label online through a 3rd party and not directly from USPS. Is this why they can do it and not get charged for a box?

      • Rockwell Sands

        Hi Bianca, this is a good question. To be honest, these shippers are doing it incorrectly, and the fact that they haven’t been charged yet is probably just luck of the draw. It doesn’t matter whether you buy the label through a third-party or from USPS directly. USPS will still charge you for a box instead of shipping a bag if they catch it…and from what I hear, USPS is really cracking down on these types of shipments lately.

        • Howard

          From experience, if you are shipping a box in a bag, you should always form fit the boxes. The risk of the bag becoming ripped or torn is significantly reduced. Keep in mind that your packages are tossed around on conveyor belts and by employees who hate their jobs. If its snug and tight, you won’t have any worries.

          • Ron

            But as I understand it, one of the requirements for ‘box in a bag’ is that the bag specifically isn’t form-fit to the box. I agree with your reasoning as to why you should keep your packaging clean and tight, but I think form-fitting the bag now makes the package ineligible for this particular rate class.

          • Rockwell Sands

            Hi Ron – you’re in fact correct about this. The “trick” here to pull this off is to protect your package contents as much as possible, all without form-fitting the bag to the box. That’s where using the right packing material comes into play!

            Simply put, if the package looks like a box, USPS is going to charge you for shipping a box.

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