USPS Large Envelope


March 13, 2024

The United States Postal Service (USPS) provides large envelopes for people in need of sending documents, magazines, and other items that don’t weigh too much. 

Sometimes, you must send a contract for signing, legal documents, or anything of value that can fit one regular-sized large envelope. USPS provides a mailing service that suits this kind of shipping need. 

What are USPS large envelopes? What are they used for? How much must you pay when sending an item using this mail service? 

This article tackles the benefits of using large envelopes when sending items to the United States. Also, it explains the First-Class Mail shipping service and Forever stamps. 

Furthermore, this write-up details the requirements of large envelope shipping, including the weight, shape, and size. 

Whenever you plan to send large envelopes via USPS, it’s best to communicate directly with the nearest post office in your area. You can visit and access a database of over 31,330 facilities nationwide.  

Whether you’re a business owner or someone planning to relocate to another area and need quick contact with the local post office,  might be an excellent resource. 

What Are Large Envelopes?

USPS offers large envelopes or “flats” for customers who need to mail documents, including legal papers, contracts, and other important files. Large envelopes are suitable for documents as boxes might be too big, while letter envelopes can be too small. 

Here are the characteristics of USPS large envelopes:

  • Flexible: Flats are rectangular and can easily bend
  • Uniformly thick: Flats must have nothing inside that can cause even a quarter-inch thickness variation
  • Have four square corners
  • Comes unwrapped, sleeved, enveloped, or wrapped

The Postal Service will treat “flats” as parcels or packages if they don’t meet the abovementioned criteria. 

What Size Is Considered a Large Envelope for USPS?

You can consider an envelope “large” if it has the following dimensions:

MeasurementGreater thanMaximum size
Height6.125 inches12 inches
Length11.5 inches15 inches
Thickness0.25 inches0.75 inches

Weight Limits of Large Envelopes or Flats

The maximum weight for “flats” or large envelopes is 13oz. USPS treats any flat that exceeds the weight limit as an item that can be shipped via Priority Mail

Let’s say you plan on using a large envelope for First-Class Mail. However, your items exceed the maximum weight requirements for large envelopes. In that case, it’ll be considered a package priced accordingly. 

Furthermore, not only does the classification change upon exceeding the weight limit. If the envelope has contents that turn it into a rigid mailpiece, it will be considered a package and is priced accordingly.

How Much Does It Cost to Ship a Large Envelope?

When you ship a large envelope, the rate increases with each additional ounce. For instance, the price for a large envelope weighing 1oz sent using USPS First-Class Mail costs only $1.35. However, if the flat weighs 13oz, you’ll have to pay the postage rate of $4.23. 

Once the flat exceeds the 13oz weight limit or if the envelope’s dimensions no longer fit the flat’s criteria, it will be treated as a package. 

You can ship these overweight items by Priority Mail or USPS Ground Advantage. Prices for both mail services will adjust depending on the package’s actual weight, size, shape, and destination. 

How Much Does It Cost to Mail a 9×12 Envelope?

Determining the exact postage stamp for your mail can be complicated. The needed postage stamp depends on the size, weight, and mail’s destination. However, in most cases, you’ll need one postage stamp for a one-ounce letter.

A legal-sized, large envelope (9in x 12in) and manila envelopes may need two stamps for the first ounce and additional postage for extra ounces. 

If you send flats via International Mail, you can use Forever or Global Forever stamps for a 1oz (or below) envelope. 

A Forever stamp is postage that can mail a 1-oz letter. This type of stamp doesn’t devalue and can always be used for a 1-oz letter, no matter how much the postage price changes in the future. 

A Global Forever stamp is the international version of the Forever stamp. You can use this stamp type when mailing a one-ounce letter anywhere worldwide. 

You can also use one Global Forever stamp to send a two-ounce mailpiece to Canada. 

How Many Stamps Does an Oversized Envelope Need?

As mentioned, the number of stamps you need depends on the size, weight, and destination of the letter you’re sending. Once your mail exceeds the weight limit for large envelopes, it will be considered a package. 

A flat envelope costs $1.35 for the first ounce and an extra $0.24 for additional ounces. So, for a 13-oz flat, you have to pay a postage of $4.23. Now, the number of stamps you’ll need depends on the value of the stamp purchased. 

In the above example, you can buy four Forever stamps to cover the four ounces and three 10-cent stamps. 

Make Sure Your Envelope Isn’t a Poly Mailer

Poly mailers are lightweight yet strong plastic bags for shipping products. It has a self-sealing lip that you fold over, making packaging tape unnecessary. You can affix your label to the back of the bag.

However, suppose you’re shipping something in a poly mailer, and the bag’s thickness is over 0.75 when lying flat and isn’t uniform. In that case, it will be considered a package. 

USPS can still consider a poly mailer within the large envelope rate if the thickness is below 0.75in and the weight is 13oz or less. 

Sending Envelopes Is Different From Sending Postcards

Envelopes differ from postcards. However, both mailpieces have their purposes. Here are some strong points of using either of the two:


  • You have more space to tell a story to the recipient. You can include a letter, brochure, or notes if they fit the envelope’s dimensions and the weight remains within the allowed limits. 
  • Receiving mail feels more personal because the mail’s contents are sealed specifically for the recipient. It gives off a sense of confidentiality and privacy. 
  • Mail feels more personalized as envelopes are more aesthetic, especially if you use actual stamps. 
  • Envelopes also promote more reader engagement, as these envelopes have to be opened for their contents to be read. 


  • You won’t miss anything with a postcard. Everything is open for everyone to see. You can use postcards to advertise or announce new products, discounts, offers, and invitations. 
  • Postcards are relatively cheaper compared to envelopes. You don’t need much time to prepare them. 
  • You can use postcards to remind people.
  • Postcards can also support a digital campaign. You can add modern file-sharing methods like QR (quick response) codes. 
  • You can quickly boost your brand using postcards. Small businesses and entrepreneurs can use postcards to promote offers. 

First-Class Mail

USPS provides a convenient way to send mail to domestic and international destinations. Also, you can send First-Class mail letters with Forever stamps that cost $0.66, good for 1oz letters. Forever stamps never expire, even if the postage rate goes up. 

Features and Pricing

Here are some features of First-Class Mail: 

  • When you send parcels via the First-Class Mail service, your letters or packages are delivered in one to five business days. 
  • The prices to deliver items weighing up to 13oz are USPS’ best offer.
  • You can include added services like Delivery Confirmation and Certified Mail. Certified Mail is a service where the sender receives confirmation through a Return Receipt service that the package was delivered.
  • You can pay one price for up to 3.5oz.
  • Prices for First-Class Mail are based on shape and weight. 


Your item is eligible for First-Class Mail letter rates if the envelope fits the following criteria:

  • The minimum measurement for a standard letter is 3.5in high x 5in long x 0.007in thick.
  • The maximum measurement is 6.125in high x 11.5in long x 0.25in thick.  

Here’s the pricing for First-Class Mail letters:

  • For standard-sized envelopes (rectangular), the USPS postage stamp price starts at $0.66.
  • For square, unusually shaped, and oversized envelopes, the stamp cost starts at $1.06. 


A postcard is a cheap way to send messages to a recipient. It doesn’t need an envelope, which is convenient. You can send postcards via First-Class Mail, but they should first fit the following criteria:

  • The postcard should be rectangular
  • The minimum measurement is 3.5in high x 5in long x 0.007in thick.
  • The maximum measurement is 4.25in high x 6in long x 0.016in thick.

The prices for postcards are the following:

  • For a standard-sized, rectangular postcard, the stamp cost starts at $0.51.
  • For oversized postcards, the stamp costs start at $0.66.

Weight, Size, and Shape Requirements

First-Class Mail letters must weigh up to 3.5oz. For large envelopes, the maximum weight is 13oz. 

All postcards, envelopes, and flats must be rectangular. If the mailpieces exceed the size and weight limit, USPS will categorize the oversized mailpieces as parcels or packages. 

Additional Charges for Irregular Mailpieces, Envelopes, and Small Packages

The following is information about the additional charge that will apply to mailpieces that don’t fit the required weight, size, and shape of First-Class Mail. 

Nonmachinable Envelopes

Nonmachinable envelopes are letters or mailpieces that the Postal Service machines can’t process. These envelopes have irregular sizes, aren’t uniform in thickness or rigid, and have unusual contents. 

Because these mailpieces can’t go through the automated process, postal workers process these items by hand. A surcharge is required to cover the extra work people do to process your package, even if it weighs less than 1oz. 

Large Mailpieces, Envelopes, and Small Packages

Letters that exceed the maximum requirements will be considered large envelopes (flats). Likewise, large envelopes that exceed the maximum requirements for flats will be regarded as a package. 

You can use USPS Ground Advantage or Priority Mail to ship packages too heavy for First-Class Mail. Aside from this, you can take advantage of other services available to Ground Advantage and Priority Mail, like Signature Confirmation, USPS Tracking, and Registered Mail. 

Signature Confirmation is the service where you can require the recipient’s signature upon receipt of the item. It also acts as proof of delivery. 

Registered Mail is a secure delivery service provided by USPS. It comes with added protection, including safes, cages, locks, and keys, to ensure the guaranteed delivery of a valuable item. 

So, whether you’re shipping large or small packages, it’s best to use an online calculator to determine the shipping cost you need to pay. provides customers with an online price calculator for their convenience. 

Compare First-Class Mail With Forever Stamps

Forever stamps and First-Class Mail have the same price at 1oz. Forever stamps allow you to use them on a 1-oz letter, no matter how the stamp prices fluctuate in the future.

For shippers to most domestic destinations, one Forever stamp is enough to send a First-Class Mail. However, if your mail is heavier, you need additional postage.

You can buy Forever stamps and other postage stamps from or partner stores like You also have the option to print postage online, which is perfect for online businesses and other eCommerce enterprises. 

Compare First-Class Package Service With First-Class Mail Flats Retail

Remember that the largest flat size can hold many items if you want to use a First-Class Mail Flats Retail. If you plan your shipment correctly, you can send many important documents, pictures, and other items without exceeding the “flats” weight limit. 

However, you may want to mind the weight of your envelope as heavier items may result in upgrading the cost of shipping into package prices. 

Let’s say your items exceed the weight limit for flats. In that case, you can choose First-Class Package Service. This mail service offers affordable rates for lightweight packages. You can learn more about this service from your local post office or the USPS website. 

Another benefit of using First-Class Package Service is the availability of USPS Tracking, Return Receipt, Collect-on-Delivery, insurance, and Signature Confirmation. However, the downside is that this package service is not eligible for refunds.

Additional Information

The following is information that you may find helpful when deciding to ship large envelopes via First-Class Mail. 

Mailing Restrictions and Prohibitions

USPS imposes restrictions and prohibitions on what people can or can’t send. These restrictions aim to ensure the safety of the consumer and the people handling the mail or package during delivery. 

Here are some items that USPS deemed shouldn’t be shipped via mail. 

  • Air Bags
  • Ammunition
  • Explosives
  • Gasoline
  • Liquid Mercury
  • Marijuana

You can also check with your local post office for package policies specific to your area. To learn the nearest post office address, visit 

For Businesses

USPS is fully aware of the need for a secure, fast, and reliable courier service. The agency is also aware of the demand for an affordable alternative for other third-party couriers like FedEx (Federal Express). 

For this reason, USPS strives to provide services that can help businesses online and offline. The Postal Service offers discounted rates for businesses using First-Class Mail. Aside from discounts, USPS offers two primary ways to help American entrepreneurs.

  • Automation discounts and commercial prices: You can get bulk discounts for presorted letters and flats for a minimum of 500 mailpieces. 
  • Data-driven shipping: Your mail becomes targetable, trackable, and measurable. Businesses can help monitor their shipping without the need for constant post office visits. 


  1. What is a Large Envelope (Flat)?
  2. First-Class Mail Fact Sheet
  3. First-Class Mail
  4. Forever Stamp Fact Sheet
  5. Sizes for Postcards