USPS Priority Mail Flat Rate Box Rules(All You Need To Know)

Have you ever wanted to know about the rules for using USPS’s priority mail flat rate boxes? It’s important to get it right, or you may get charged additional mailing fees.

Anything that you wish to put in a flat rate box must fit in an unmodified version of this box, without needing to use extra cardboard or tape to seal the box up. It must also weigh less than 70 lb.

Today we will talk about all the necessary requirements when it comes to flat rate boxes.

What Are The Priority Mail Flat Rate Box Rules?

USPS provides free shipping boxes for its Priority Mail Flat Rate service, and these are a great thing to make use of if you are a USPS customer. However, you must make sure that the package:

  • Will seal easily with the contents inside, without depending upon excessive tape to hold it shut (a bit of tape for security is acceptable)
  • Does not weigh over 70 lb.
  • Is not warped so much that the flaps do not fold into the intended slots

A flat rate box is a great way to mail items that will fit inside its dimensions. These vary depending on the box that you choose. You can pick small boxes, medium top-loading boxes, medium side loading boxes, and large boxes.

Choose whichever suits you best, but bear in mind that the cost of mailing goes up for larger flat rate boxes.

What Is Allowed In A Flat Rate Box?

You can mail anything legal in a flat rate box. USPS has a list of excluded items on their site, and these are excluded from both flat-rate mailing services and normal shipping services.

A few examples include:

  • Alcohol
  • Guns
  • Many kinds of tobacco
  • Batteries
  • Cleaning products
  • Dry ice
  • Prescription medications

There are other prohibited items, so make sure you check USPS’s website if you are concerned about something you are shipping.

How Do I Ship A Priority Mail Flat Rate Box?

You can order the boxes free of charge from the USPS website, or collect one from your local post office. You should pack the items in one, padding anything fragile to avoid damage.

When this has been done, you can purchase a pre-paid label from the USPS website and stick it to the parcel. You can then request for USPS to collect the package from your home, or drop it at a local post office.

If you don’t want to buy your label online, simply take the package to your local post office and ask the shipping clerk to price it for you. Pay for the shipping, and give the package to the employee.

Related Post: USPS Priority Mail Flat Rate Envelope Rules(A Must-Read)

Q&A Section

Q: Are USPS Priority Mail Flat Rate Boxes Free?

Yes, these boxes can be obtained for free from USPS. They provide them as part of their service to their users, and you can either pick one up from your local post office or order them from the USPS website.

However, you will have to pay to mail them – only the box is free.

Q: Can Flat Rate Boxes Bulge?

Yes, it is okay for a flat rate box not to be completely flat. Remember, “flat rate” refers to the shipping method, and not to the shape of the package.

It means that you will pay a flat fee for your package, no matter where you mail it to or what it weighs.

However, your package should not bulge excessively. If it will not stay shut without significant amounts of tape or if you have to add extra cardboard to compensate for the bulge, it will be rejected by USPS.

Do not try to warp your flat rate box out of shape, because you may end up getting charged extra shipping fees, or being unable to mail the package at all.

Q: Can I Use Duct Tape On A Flat Rate Box?

No, you can’t. Duct tape is specifically prohibited by USPS, but not because it makes the box too big. It is banned because it does not stick well to the cardboard that the box is made from.

Duct tape adheres well to itself, but it will not hold up well if you use it to pack your parcel. Because cardboard is made up of loose fibers, duct tape will simply peel off.

For this reason, many couriers, including USPS, have banned it. It caused too many issues with lost mail and damaged packaging.

Instead, use proper packing tape, designed for shipping items. Brown tape is a good option, or you can buy clear packing tapes if you prefer.

Related Post: Can You Use Duct Tape On Packages?(Not Really)

Q: Can You Use Any Box For Priority Mail?

You can use any box if you only wish to use Priority Mail, and not Priority Mail Flat Rate. Your box must have “Priority Mail” written on it in large, clear lettering, or a Priority Mail label prominently displayed.

You cannot use your own box for Flat Rate mail, because USPS cannot check that your box conforms to their stipulations about size. Because Flat Rate is shipped based on dimensions, you must use USPS’s boxes.

Q: Is It Cheaper To Use Your Own Box For Priority Mail?

It can be cheaper, yes. In some Zones, for light items, using your own box and shipping using Priority Mail is more economical than sending an item using the Flat Rate service.

Online shipping calculators may help you determine which option is better. Make sure you compare the different rates carefully to get the best deal.

Q: Can Priority Mail Boxes Be Reused?

Yes, provided that the box is still going to be used for Priority Mail shipping. You cannot use it for standard shipping or any other service, even if you hide the “priority mail” label.

These boxes are only to be used for Priority Mail items because USPS provides them for free for that purpose. Misusing them is a violation of the law.


As long as an item is under 70 lb., small enough, and permitted by USPS, it can go in a Priority Mail Flat Rate box. Make sure you choose a box that your item will fit into.