Square Envelope Postage


March 12, 2024

Sometimes you want to do something fancy with your mail. Instead of sending your “special” correspondence like invitations in regular envelopes, you add a bit of creativity and ship them in square envelopes instead. 

So, what are square envelopes, and how much do square envelope postages cost? Also, are there additional regulations to consider when sending special correspondences, like wedding invitations, using square envelopes? 

This article explores square envelopes and how the United States Postal Service (USPS) handles nonstandard-sized mailpieces. Also, it provides tips on preparing special letters like wedding invitations, especially if you plan to ship them via USPS. 

Furthermore, this writeup introduces First-Class Mail as one of the primary mail services you can use when sending invitations. 

Whenever you plan to send mailpieces via USPS, it’s best to have direct contact with the nearest USPS facility in your area. You can visit FindPostOffice.org and access a database of over 31,330 facilities nationwide. 

Our website can benefit your business, especially if you need access to post offices. It could also be extra helpful if you plan to relocate to another area and need quick contact with the local post office.  

Weight, Size, and Shape Requirements

When you want to fancy up your letters and go with a nontraditional rectangular envelope, a square envelope might be your choice. USPS provides postage prices for envelopes of different shapes and sizes. Here are some square envelope postage shape, weight, and size requirements.

Square Envelope Sizes

Let’s say you’re planning to send wedding invitations, and the invites you want to send are square-shaped. USPS allows people to send these kinds of custom-shaped parcels.

The standard sizes for square envelopes are easy to remember because all sides have the exact measurement. Sizes range from 5-inch squares (5 x 5in) to 9-inch squares (9 x 9in). 

Odd-Sized or Nonmachinable Letters

For USPS, any nontraditional-shaped mail is odd-sized. These envelopes include square, vertical, lumpy, rigid envelopes and mail with clasps, buttons, or strings. 

Odd-sized envelopes are treated as nonmachinable letters, even if they weigh less than 1oz. Furthermore, unusually shaped mail must be hand-canceled. 

Hand-canceling means that each stamp on your envelope is marked canceled using a hand stamp instead of a machine. Hand canceling prevents odd-sized, nonmachinable envelopes from getting damaged by the post office’s machine. 

Remember that there’s a standard measurement that’s machinable according to USPS. For First-Class Mail, the standard sizes start at:

  • Minimum: 5 inches long x 3 1/2 inches high x 0.007 inches thick 
  • Maximum: 11 1/2 inches long x 6 1/8 inches high x 1/4 inches thick

If your invitations fall between these measurements, they’re considered First-Class Mail letters. However, your invites may be counted as packages if they exceed this size. You may have to consider sending invites via the First-Class package service. 

Use a Non-machinable Surcharge Stamp

Oddly-sized envelopes are considered nonmachinable, so a surcharge is added to First-Class Mail. This extra fee is needed because non-standard mailpiece sizes don’t go through a local post office’s automation process. Packages are sorted and processed by hand, which can be labor-intensive.  

Features and Pricing

Square envelopes are usually sent via First-Class Mail. This mail service delivers letters in one to five business days. The best items for this mail service are up to 13oz. Also, First-Class Mail can include a Delivery Confirmation service, similar to Certified Mail

The pricing feature of this type of mail service for square, oversized, or oddly-shaped envelope stamps starts at $1.06. Additional charges may apply depending on the shape and weight of the letter. 

Which Postage Rates Changed?

USPS recently announced a price hike, increasing the prices of many of its mail service rates. 

You can send odd-sized envelopes like square letters via First-Class Mail with nonmachinable surcharges. Currently, the price of sending a letter via First-Class Mail is $1.06.

Why Square Envelope Postage Costs Some Extra Cents

The square envelope has a surcharge because the oddly-shaped parcels are not processed automatically. Instead, these items are sorted and processed by hand, making the sorting tedious. 

Additional Charges for Irregular Mailpieces, Envelopes, and Small Packages

There are additional charges if your mail falls under these conditions:

  • When the envelope is unusually shaped, rigid, and lumpy   
  • When the mail has clasps and is nonmachinable
  • When the letter exceeds the dimension limits of First-Class Mail
  • When the large envelopes (flats) exceed the maximum dimensions and are rigid, not uniformly thick, and non-rectangular

How to Know How Much Postage You Need?

Determining the actual postage required to send customized letters in odd-sized envelopes can sometimes be confusing. Postage is integral to the functioning of the Postal Service, so paying the correct amount is essential. 

Postage is the amount you pay for USPS to deliver your letter to its intended destination. On the other hand, a postage stamp acts as proof that you’ve paid the postage amount associated with the letter. 

Do Square Envelopes Require Extra Postage?

As mentioned, any envelope with a shape other than the standard size is considered odd-size or nonmachinable. When a package’s size is not the standard, it might not get processed by USPS automatically. 

A square envelope will cost an extra $0.40 per letter, on top of the postage needed to cover the dimensions and weight of the envelope. 

For instance, a 1oz letter in a square envelope needs a $1.06 stamp instead of a $0.66 one. Meanwhile, for wedding invitations in square envelopes weighing 2oz, you’ll have to pay $1.30 (versus a $0.90 stamp).  

You can also visit USPS.com and use the USPS postage calculator to estimate how much you must pay for your mail. 

Why Do Square Cards Require Extra Postage?

Customers may be surprised that the envelope’s shape can cause an additional surcharge, even if the letter has no additional ounces. As mentioned above, the extra postage is due to the difficulty of processing non-standard envelope sizes. 

The machines used for automation would often jam when differently sized letters are fed into the machine. So, when you pay the extra amount of postage for square cards or envelopes, you’re paying for the additional work done by postal workers to process your mail. 

General Tips for Applying Postage to Mailing Invitations

Sending invitations via mail is simple once you understand what to do. Remember that size and shape matter when you ship invitations via USPS. 

As mentioned above, and according to the USPS website, using square envelopes entails extra costs. So, it’s best to decide first if you’re willing to spend extra for custom-designed invitations with non-standard envelope sizes. 

When in Doubt – Add Extra Postage

If you’re unsure how much postage you must spend, here’s a tip. You can always add extra postage or go with the higher rate when in doubt. You can check the official USPS price list or use a postage calculator to ensure you get the exact amount. 

Pick the higher postage rate to be sure. You won’t spend too much, as the rates are relatively affordable. 

Order Your Postage in Time – Or Go on the Hunt!

If you’re getting married, don’t do your wedding planning at the last minute. Plan things to avoid getting stressed a day or two before the big day. This advice also applies to sending out invitations. You should order your postage in advance, especially if you get them from websites like USPS.com. 

Remember the delivery time for USPS stamp orders, which usually arrive within one to two weeks or, in some instances, longer. So, if you’re to order postage, you should consider this delivery delay. 

You can also find other stores to buy postage stamps for invitations. You may also need to visit other post offices to get the required stamps. Go to FindPostOffice.org to access the location of different USPS retail facilities nationwide. 

Protect Your Envelope

You can add fancy stuff to your envelope, like wax seals. However, you must protect it as letters go through machines when processed in a USPS facility. Automated machines can break your wax seals. 

Remember that USPS embraced automation to speed up the delivery process. However, the downside is that letters may get damaged, especially if their size differs from the standard. 

It’s advised to use double envelopes when using wax seals. You can also use a clear sleeve to protect your invitations. Remember to place the stamp on the sleeve so it can still be canceled. 

Don’t Forget the RSVP Envelope!

When sending an invitation, you may want to get a response from your guests if they’re going or not. Though your guests can simply phone you their reply, sending a response letter is much more endearing. 

You can add to your invitation an RSVP (répondez, s’il vous plaît or respond, if you please). An RSVP envelope has an extra postage stamp attached for ready use when your guests want to send their response. Using a 1oz Forever stamp for an RSVP envelope is recommended. 

Mail Will Still Get Lost

Though USPS tries its best to provide security to millions of Americans, there are always instances where mail gets lost. Some of the invites you’ve sent via the Postal Service may get lost. So, it’s best that you have extra invitations to cover this problem. 

Mail in 1 to 5 Business Days; Small Packages in 2 to 5 Business Days

When sending mail via USPS, you can choose different mail services. However, each service has varying delivery times. For instance, if you ship greeting cards via Priority Mail Express within the U.S., your mail can be delivered the following day or the next two days. 

If you use First-Class Mail, it would take one to five business days to arrive. For small packages, you can choose USPS Ground Advantage to have your small parcels weighing up to 70lbs delivered within the United States in two to five days. 

You can choose First-Class Mail International for deliveries outside of the U.S. However, delivery time may depend on the destination country. However, you can have your invites delivered to over 180 countries. 

Additional Information

The following is additional information you may need to know when sending invitations in square envelopes via First-Class Mail. 

Mailing Restrictions and Prohibitions

You may have things you want to send to people. Still, USPS has specific regulations and standards that may prevent you from sending anything via mail. For instance, USPS provides particular regulations for sending hazardous materials like smokeless tobacco or cigarettes.

On the other hand, there are items that you can’t send via mail. For example:

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

You can also check with your local post office for regulations specific to your area. Find the address of the nearest post office by visiting FindPostOffice.org. 

For Businesses

USPS knows the importance of speedy, secure, reliable, and affordable courier service. For this reason, it strives to provide business-friendly services and resources to help entrepreneurs grow. USPS provides discounted rates specifically for businesses that use the First-Class Mail service. 

USPS offers two ways to help U.S. businesses send letters via First-Class Mail:

  • Data-driven shipping: Via First-Class Mail, your package is targetable, trackable, and measurable. The service can help businesses monitor their deliveries, make address corrections when needed, and have access to forwarding options.
  • Automation discounts and commercial prices: You can get bulk discounts at commercial pricing for presorted letters and flats with a minimum quantity of 500 mailpieces. You can also have automation discounts if applicable. An example of an automation discount is the “one price up to 3.5oz” discount. 

There are also perks like promotional messages with bills, statements, and invoices. However, “one price of up to 3.5oz” is not a blended rate and doesn’t apply to First-Class Mail single-piece letters.


  1. What envelope size requires additional postage?

You can calculate USPS First-Class Mail prices by considering the package’s shape and weight. So, if the shape and weight exceed a particular mail category, you must pay additional postage.

  1. How do I know how much vintage postage I need?

Vintage postage is any kind of stamp that’s no longer in circulation. However, you can still use vintage postage stamps to design and make your wedding invitations look aesthetically pleasing. 

Let’s say you’re sending invitations and mailing using a square envelope that weighs less than 1oz. 

You may ask, “How many Forever stamps do I need for a square envelope of this size?” The answer is you can use a 2-oz Forever stamp. 

Note that the heavier your invitation, the costlier it is. For instance, if you plan to make your invitations out of cardstock or cardboard, you may have to weigh your mail again and provide additional postage. 
Suppose you’re unsure how much you must pay for your invitations. In that case, you can head to your local post office and get an estimate of your total mailing expenses.


  1. First-Class Mail
  2. Recap: First-Class Mail Letters
  3. Sizes for Letters
  4. Mail & Shipping Services
  5. First-Class Mail International