Have you ever dropped a letter in the mailbox, only to have it returned days later with a mysterious message about “non-machinable postage required”? If so, you’re not alone. The U.S. Postal Service (USPS) has a lot of rules—especially about what types of mail can go through its sorting machines. That’s where the non-machinable stamp comes in.
This article breaks down everything you need to know about non-machinable stamps in simple terms: what they are, when you need one, why they cost more, and how to use them correctly. Whether you’re sending a wedding invitation, a square-shaped birthday card, or just a letter that’s a bit bulkier than usual, this guide will help make sure your mail gets where it needs to go—without hiccups.
What Is a Non-Machinable Stamp?
Let’s start with the basics. A non-machinable stamp is used when your mail can’t be sorted by the machines at USPS sorting centers. Those machines are designed to handle letters that are standard in size, shape, thickness, and flexibility. But not all mail fits the mold. If your letter is rigid, square, oddly shaped, too thick, or has something bulky inside, it can’t safely pass through the automated system.
When that happens, USPS has to sort it by hand—which costs extra. That’s what the non-machinable surcharge covers. It’s not a different type of stamp, technically, but it adds extra postage to account for the manual labor. And yes, there are special stamps you can buy that cover both the regular First-Class rate and the non-machinable surcharge together.
Why Some Mail Can’t Be “Machined”
The USPS relies heavily on automation to keep costs down and delivery fast. Their machines are fine-tuned to process envelopes and cards that meet very specific requirements. If your mail doesn’t meet those standards, it might jam the machines or get damaged during processing.
Here’s what USPS considers when deciding if something is “machinable”:
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Shape: Standard letters must be rectangular.
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Size: They should be at least 5 inches long and 3.5 inches high.
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Flexibility: The item must bend easily and evenly.
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Thickness: Uniform and under ¼ inch.
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Weight: Not more than 3.5 ounces for First-Class letters.
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Surface: Should be smooth, with no buttons, clasps, or strings.
Anything that doesn’t meet one or more of these rules is classified as non-machinable and needs extra postage.
Real-Life Examples of When You’ll Need One
Not sure if you need a non-machinable stamp? Here are a few everyday examples that will help you decide:
1. Square Envelopes
They might look chic for wedding invites or birthday cards, but square envelopes don’t fit into USPS machines. Even if they’re light and bendy, they’re still non-machinable.
2. Bulky Greeting Cards
Ever buy a card with raised embellishments, pop-ups, or a button that says “press here”? Those fun features make the envelope rigid or uneven—and that means it needs manual handling.
3. Handmade or Heavily Decorated Mail
Crafty mail, like scrapbook-style envelopes or hand-decorated cards, often end up being thicker or less flexible than standard ones.
4. Bubble-Wrapped Items
Sending a small pin or sticker in a padded envelope? Even if it’s flat, the bubble padding makes it too thick and squishy for machines.
5. Multiple Inserts
Stuffing more than one sheet or a few photos into a letter? It might still be under 1 oz., but the uneven weight distribution can affect how it travels through machinery.
If your mail falls into any of these categories, it’s a safe bet that you’ll need to add the non-machinable surcharge—or use a stamp that includes it.
What Does a Non-Machinable Stamp Look Like?
Here’s a fun fact: there’s no stamp that literally says “non-machinable” on it. Instead, USPS releases special stamp designs each year that are priced to include the surcharge. These often feature butterflies and are officially called “non-machinable surcharge stamps“ in USPS terms.
For example, the Butterfly Stamp is commonly used for non-machinable mail. It currently (as of 2025) costs $0.88, which includes:
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$0.41 for the First-Class 1 oz letter rate
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$0.47 for the non-machinable surcharge
Of course, you don’t have to use a butterfly stamp. You can just use regular Forever stamps and add enough to meet the total required postage.
Where to Buy Non-Machinable Stamps
Getting your hands on non-machinable stamps is easier than you might think:
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Post Offices: Ask at the counter for “butterfly stamps” or “non-machinable rate stamps.”
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USPS.com: You can order specific denomination stamps online.
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Retail Stores: Big-box retailers or office supply stores often sell them in stamp booklets.
If you can’t find a stamp with the exact price, you can always mix and match standard stamps to meet the total. Just make sure you calculate the correct amount!
How to Know If You’re Using the Right Postage
Worried your letter might not make it? Here’s a quick checklist to see if your envelope needs a non-machinable stamp:
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Is it square or an unusual shape?
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Does it feel stiff or rigid?
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Is it thicker than ¼ inch?
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Does it have lumps, buttons, or raised elements?
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Is it less than 5 inches long or 3.5 inches tall?
If you answered yes to any of those, use a non-machinable stamp—or risk having your mail returned for insufficient postage.
Pro tip: If you’re ever unsure, bring your envelope to the post office. They can weigh it, measure it, and let you know exactly what postage is needed.
Why the Extra Cost?
It’s easy to feel annoyed about paying more just because your card is a little thicker or square. But when you understand why the surcharge exists, it makes more sense.
Mail that can’t go through machines has to be sorted by hand. That means someone at the USPS has to manually feed it, process it, and ensure it gets to the right place. That takes time and labor, and the surcharge helps cover those extra costs.
Plus, using the correct stamp helps your mail avoid delays or even being returned.
Can You Avoid the Surcharge?
Yes—sometimes. If you want to avoid paying extra, you can:
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Use standard rectangular envelopes.
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Keep your mail flexible and under ¼ inch thick.
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Skip any bulky or decorative items.
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Weigh your mail carefully to stay under the 1 oz. limit.
That said, if you’re sending something special or decorative, paying a little more for proper handling is worth it to make sure it gets delivered in one piece.
What Happens If You Don’t Use the Right Stamp?
If you put a regular Forever stamp on a non-machinable item, a few things could happen:
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USPS might deliver it but send the recipient a “postage due” notice.
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It could get delayed while they figure out what to do.
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Worst-case scenario: it gets returned to you.
None of those are ideal—especially if your mail is time-sensitive or for a special event.
Final Thoughts: Better Safe Than Sorry
Mailing a letter may seem simple, but not all letters are created equal. The non-machinable stamp plays a vital role in helping USPS handle the mail that’s just a little different from the rest. Whether it’s square, stiff, or stuffed, your envelope deserves the extra attention it needs to arrive safely.
So the next time you’re sending out an artsy card, a thick invitation, or anything unique, don’t forget to ask yourself: Is this non-machinable? If the answer is yes, reach for that butterfly stamp—and give your letter the extra care it needs to get where it’s going.