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!

41 Comments

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  1. Concerned Customer

    Lately, I’ve received a lot of damaged or leaked items from “box in a bag” shipments. The result is a missed sale and a dissatisfied customer, since these items must be returned because of damage. Polymailers are great for the intended use ( such as folded clothing); but for gift items that need to have the product packaging intact; or liquids and cosmetics that often leak during shipping, it’s not such a good idea.

    Reply  
  2. Where does one buy these polymailers & are they plain or printed

    Reply  
    • Rockwell Sands

      Hi Sandra – you can buy these poly mailers at either ULINE or Amazon! I personally use these plain ones that are 13×10″ from Amazon.

      Here’s a link to some on the ULINE website too: https://www.uline.com/Grp_191/Poly-Mailers. You can find any size and color you want there!

      Reply  
    • Bunnie

      Hi Sandra, if you are in need of it quickly WMT retailer have poly on the shelf in several sizes. Also dollar tree offers several options including a bubble lined poly. ANY poly bag you have received in a shipment to you, is reusable provided you can make the corners return to their original shape. I open all my poly’s with care with deliberate intention to reuse (and retape as the original seal is not salvageable). Remember the sweet spot to make poly worth your while is if the item fits and the “pointy pillow corners” of the poly are not stretched or misshaped. They must appear just as they do unfilled when you first buy the poly (do not wrap an item in poly like a gift or present). Loose fitting. Once that corner is no longer visible you have defeated the cubic rate grace allowance and you got back to 3 dimension equation UPCHARGE to your account. It may happen up to 5 weeks after your delivery!!! Or package returned to you. Don’t proceed unless you comprehend. Pirate cs will accept a photo of your package to assist you if you doubt yourself. I had to several times, until it clicked. Be well.

      Reply  
  3. gina d karaba

    hi I found that I want to ship a box inside a bag simply so the small package doesn’t get lost. But I’m curious how big a box I can out inside. I also never get the option to put only 2 dimensions in. I’ve just shipped a 5 x 5 x 5 box inside a bag but I had to pay MORE to put it in the bag because it weighed more. ?

    Reply  
    • Rockwell Sands

      Hi Gina – there’s no real answer for the maximum size of the box you can put in…but the maximum size envelope you can use to qualify is 18×18″. When entering your info, you’ll need to select “Envelope or Poly Mailer” instead of a box, that way you’ll have the option to only input two dimensions (and these will be the dimensions of your outside envelope). I hope this makes sense!

      Reply  
    • Bunnie

      If you are on Pirate IT IS UP TO YOU TO CHOOSE the method of package!! They assume it is a box. So your job is to tell the form you are using a poly bag. On the page where you see RIGID BOX at the top, click the arrow pointing down on the right side. That click will open a lot of options. Once opened click on: Envelope, Padded Envelope, Poly Bag, Soft Pack, or Box in a Bag. From there you enter the POLY BOX dimensions which only allow 2 numbers! note: Max is 18″ by 18″ so you must enter under that measurement. You can still use an 18″ by 18″ poly, but enter the dim as 17.50 by 17.50 or less depending on your contents. Your example of 5″ cube would fit using a much smaller dim making sure the pointy corners of the poly are noticeable not formed to your 5″ cube.. Hope that helps!

      Reply  
  4. Shanandoah McQueen

    So why not just put the product in the bag and leave the box out? Wouldn’t that be even more cost-effective?

    Reply  
    • Bunnie

      The main thing here is the grace of 20 lb weight limit and the allowance to only pay for a thin package material covering of a poly bag and not a 3 sided measure box. The limitations of an economical rate in the flat category was your only option with out cubic, and flat rate must fit in those specific sized boxes. The poly bag offers an option for that gap. Also some things shape or weight would be difficult to maneuver if not confined in a box, in the bag. Somethings need the rigidity and protection of a box to arrive in the proper shape in which you expect. Somethings have a sharp edge that may break through the poly bag. If you can bubble wrap a shape and protect it with padding then by all means just use the poly. If you are shipping a teddy bear stuff it in a poly bag. If you are shipping a ceramic bear, bubble wrap and pad it, place it in a box then put that in a poly-but allow the poly corners to be seen, not stretched over the box as in wrapping a gift. Hope that helps.

      Reply  
      • Sabina Charles

        I was VERY surprised to see that this method was suggested for perfumes. According to USPS the ONLY way fragrances can be sent is Ground shipping as they are considered hazardous material and if not sent ground it is subject to fines.

        Reply  
    • Rockwell Sands

      You could do this! But a lot of shipments only get sent in cardboard boxes, like in the case of subscription boxes or fragile items that need protection. The “box in a bag” method if mostly for shippers who need to send their packages in boxes to try and save even more money by putting their packages inside of poly mailers. Hope this helps!

      Reply  
  5. Bunnie

    Eureka! I just got it! I couldn’t wrap my mind around why the poly bag lowered my rate so substantially, when my weight remained the same. I think I may be able to help clear up the box in a bag fog.
    So the reason this cubic rate is SO attractive is it takes only TWO dimensions of your “POUCH” shaped package. Length and width. (think of a pillow). Conversely the measurement for a box takes THREE dimensions. Length width and height. So…In order for your package to qualify for the pouch (cubic) rate, and not the box rate, it must Look like a pouch. It has to convey it is pouch. Ask yourself from across the room do you see a pouch or a box? If YOU SEE a box, then the PO will see a box. And very likely while handling it, they will up charge you for THREE dims!
    What the pirates are trying to get across is A, you can’t wrap your box like a birthday gift. (that will look like a box). B. You can’t shove a box in a poly and have it stretched to capacity (that will also look like a box)
    If you are putting a box in a poly mailer, be sure the bag is oversized enough, or loose and extends beyond the boundary of the recognizable shaped box contained within. The poly bag purpose is to DELETE the 3rh dim measurement of the box and look like a stuffed pillow. Again from across the room do you see a pouch? Or a six sided box? If you see a box they may re measure your box which will add that 3rd dimension and tag your account for the full counter priced rate.
    If you don’t have enough loose poly bag under 18″ max on either side (of a box), cubic rate is not for you. AND…if your item doesn’t need to be in a box, YOU ARE GOLDEN. SHIP the football, rain boot, teddy bear in a poly bag…it has no sides once in a bag with 2 dimensions!!! Hope that helped!

    Reply  
  6. Kris

    Does anyone know where I can find this in the Domestic Mail Manual DMM?

    Reply  
  7. Kris

    Had a man come into our post office last night. The clerk he went to noticed he had cubic rate prices on his packages but they were only 2 dimensional. She thought this was odd because cubic is usually more than 2 dimensions. The man said that because they were bagged they fell under the box in a bag cubic pricing which only requires the lengh and the width. He said he bought these bags specifically for this method of shipping. The problem I saw was that he had taped the bags so that they form fitted the box. I did not know at the time about box in a bag cubic shipping but to me they looked like wrapped (not bagged boxes). The clerk that was helping him explaned that his accout would be charged if the system found any problems between his shipping label and packaging method. He said that he had been shipping them that way for years and not had a problem. The clerk let him know that he had just been lucky. He got upset and said he would just take his packages to another post office were it won’t be a problem. I’m sure if he checks whatever account he pays his shipping with he will probably see some extra shipping charges. Maybe not for all but for some. Whatever you do don’t defraud the usps because it will eventually catch up with you and it could result in felony charges depending on the amount at the very least a big hassle with the postal inspection service which is a federal law enforcement agency. Thanks for the info on this page I am going to share it at our post oftice so we know what box in a bag cubic pricing is and how it works.

    Reply  
  8. Custom Design Boxes

    Outstanding.. your outlook on this is amazing

    Reply  
  9. Joan Recker

    How do you calculate shipping charge on a poly bag?

    Reply  
    • Rockwell Sands

      Hi Joan – the cost to ship depends on the weight of your bag, its dimensions, and how far away you’re sending it. I usually use a simple rate calculator to see how much a package will cost to ship. Here’s one that I use all the time that you can check out: http://www.pirateship.com/rates. Just type in all the info for your bag, and it’ll show you the cheapest way to ship it!

      Reply  
  10. Tee

    Hi there. I need some advice. I’d like to ship my orders in 9x6x4 inch LWH. Can you recommend a msg for me? Thanks in advance😊

    Reply  
  11. Robin

    Hi, when I put the dimensions of 18″ x 18″ poly mailer in Pirate Ship Box in a Bag setting, the price page shows the regular Priorty Mail rate instead of the Cubic Rate and doesn’t give the option to even see the price of the Cubic Rate for an 18″ x 18″ mailer. If I change the parameters to 18″ x 16″ for instance, it will give the cubic rate price. Do you know why that would be? Thanks.

    Reply  
    • Rockwell Sands

      Hi Robin – I think that’s because you might be at the maximum dimensions there, for cubic Box in a Bag. The max dimensions is 36″ inches in total (length + width), so you’re coming up right on that threshold…and regular Priority Mail may actually be cheaper. Have you tried buying the label with a smaller dimension, like 17.5 x 17.5″? That may help you lock in the cubic rate! Hope this helps.

      Reply  
  12. Nancy

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

    Reply  
    • Rockwell Sands

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

      Reply  
      • Jack

        Please, please, please use a grocery bag. The more people that ship incompetently makes me look better as a seller. Send it off in a grocery bag and make me more money.

        Reply  
    • 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.

      Reply  
  13. 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

    Reply  
  14. 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?

    Reply  
    • 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.

      Reply  
  15. Travis

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

    Reply  
    • 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 🙂

      Reply  
    • VICTORIA MARGARET

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

      Reply  
  16. Sammy

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

    Reply  
  17. Ahlora

    Thank you for clarifying this!

    Reply  
  18. 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.

    Reply  
  19. 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?

    Reply  
    • 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!

      Reply  
  20. 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.

    Reply  
    • 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?

      Reply  
      • 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.

        Reply  
        • 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.

          Reply  
          • 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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