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USPS Flat Rate Envelope Restrictions

usps flat rate envelope restrictions

Do you want to send a Priority Mail flat-rate envelope with the United States Postal Service?

These envelopes come in several different sizes and can be sent for a single low flat rate. This essentially means that the cost of sending the mail is determined by the type of envelope you choose. The great thing about this service is that it is easy to work out how much postage you need to pay. However, it is important to take the terms and conditions of using this service into account.

Let’s find out more in my in-depth guide to the USPS flat rate envelope restrictions.

About USPS Flat Rate Envelopes

You can pick up these envelopes free of charge from your local post office branch. The postage fee is based on the type of envelope you are using, and the maximum weight is seventy pounds. The fee does not vary according to the distance, making this a great option when sending mail to another state.

Available Types

There are six main types of USPS Envelopes to choose from when you choose the flat rate service. The type you choose will mainly depend on what you want to send. Let’s take a closer look, including the cost of sending them.

Priority Mail small 

These envelopes measure 10 inches by 6 inches with the flap closed. They are ideal for sending letters, cards, passports, and small documents. The postage rate when choosing this option is $8.70.

Priority Mail Standard

This option is slightly larger and measures 12.5 inches by 9.5 inches with the flap closed. This is a good choice if you want to send clothes or documents. The rate for sending this type of envelope is also $8.70.

Priority Mail Window Small

You can use this option to send small documents such as passports and letters. The measurements are 10 inches by 5 inches with the flap closed. The postage charge is slightly higher at $8.70.

Priority Mail Window Large 

Even though this option is slightly larger, the postage cost is also $8.70. It measures 12.5 inches by 9.5 inches with the flap closed. This is a good choice for sending large pre-addressed envelopes and full-page documents.

Priority Mail Padded 

You will be charged $9.30 when you choose this option. It is designed for any items that need a little extra padding when being mailed. The dimensions are 12.5 inches by 9.5 inches with the flap closed.

Priority Mail Legal 

This item measures 15 inches by 9.5 inches with the flap closed. This is the perfect size for sending legal size documents. The standard postage is $9.

How To Attach Postage?

Now you know exactly what the postage rate is, sending this type of mail is pretty easy. There are several different options for purchasing and using postage for these envelopes. Let’s take a closer look at the main options so that you can choose the best one for your needs.

How To Attach Postage 


The postage price is dictated by the type of envelope you need. Once you have worked out the postage, you can choose stamps that perfectly match it. They will make up the postage and attach them to complete the process. 


The United States Postal Service offers a Click-N-Ship service that allows you to buy and print postage at home. As long as you have a printer, this will save you from having to go to the post office and purchase stamps. Simply pay for the postage online, print your label, and arrange for the item to be collected. 

Prepaid labels

You can purchase a prepaid label and attach it to your envelope. These labels are sold by a large number of companies and make sending mail more convenient. All you need to do is pay for the label, print it out, attach it, and send the item. 

Sending the item is very easy. All you need to do is pop the item in an official United States Postal Service box. This saves you from having to make a trip to the post office or waiting for the item to be collected. 

Maximum Dimensions And Weight

It is important to make sure you choose the correct flat rate envelope for your needs. The maximum combined girth and length of the package is 108 inches. This means that you should measure the package around the widest part and add this number to the length.

If your package is too thick, it will not fit through the slot in the post office box. You also need to make sure that the flaps can be closed within normal folds. If you have to use a large amount of tape, you are likely to be charged extra to send the parcel. 

Domestic and international are different…

The maximum weight of the item you are sending is 70 pounds for domestic mail. However, the maximum weight is reduced to just four pounds if you are sending it internationally. It is a good idea to weigh the package to make sure it doesn’t exceed the weight restriction. 

Items That Can Be Sent In A Flat Rate Envelope

As long as you keep the weight restrictions in mind, you can send most types of items in this way. They need to fit well without stretching the envelope. While bulges are permitted, you should not have to use an excessive amount of tape to seal the flap.

The general rule of thumb is that ‘if it fits, it ships.’ This means that the flap should close easily. If the seal is strained by the contents, you should choose another shipping option. 

Items That Can Be Sent In A Flat Rate Envelope

Acceptable items include:

  • Documents
  • Letters
  • Clothes
  • Shoes
  • Toys
  • Pillows
  • Metal art pieces
  • Photographs
  • Books
  • Blunt kitchen utensils

Items That Cannot Be Sent In A Flat Rate Envelope

It is important to note that certain items cannot be sent in this way. Some of these are prohibited, while others are restricted. It is best to consult a United States Postal Service employee. 

Prohibited items include:

  • Ammunition
  • Explosives
  • Airbags
  • Gasoline
  • Marijuana 

Restricted items include:

  • Hand sanitizer
  • Alcoholic beverages
  • Paint
  • Lithium batteries
  • Perfume
  • Matches
  • Glue
  • Cremated remains
  • Hemp products
  • Perishable items
  • Nail polish
  • Dry ice

Information You Need To Know About USPS

We’ve put together a superb collection of guides when posting or waiting for a delivery! Let’s start with Where Can I Drop off USPS Packages, as well as Can I Use Poly Mailers For USPS? Or, for official documents, you’ll want to know Does USPS Notarize Documents, along with How Long Does USPS Forward Mail and Does USPS Forward IRS And Other Government Mail?

You might also be wondering Do USPS Stamps ExpireWhat Does Insufficient Address Mean At USPS, which is quite common, and once you’ve posted your parcel, check out When Do USPS Trucks Go Out For DeliveryDoes USPS Deliver On Saturdays, and What Is USPS Informed Delivery in 2023.

Tracking your parcel will also give you peace of mind! So, learn more about USPS Priority Mail vs First Class, followed by How Fast IS USPS First Class, and you’ll need to know Does USPS Have First Class TrackingCan You Track A USPS Package Without A Tracking Number, or perhaps, Why Is USPS Tracking Not Updating?

Final Thoughts

Using a Priority Mail Flat Rate Envelope is an easy way to save money on the cost of postage. All you need to do is choose the correct type of envelope for the item you want to send. Then just make sure you attach the right amount of postage.

However, there are specific size and weight restrictions that you need to be aware of, as well as a few items that cannot be shipped or are restricted in some way. But no problem, I’ve covered them all in this article, so simply refer to it whenever needed.

Good luck, and I hope your letter arrives safely!

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About Calvin Miller

Calvin has over 20 years of experience in a leading employment agency in Tucson, Arizona, and is passionate about finding the perfect position for everyone who uses the company’s services.

He lives with his wife, Annette, and their two sons Jake and Joseph.

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