Non-machinable stamps are postage stamps priced to cover a standard First-Class letter plus a surcharge for envelopes that can’t run through USPS automated sorting equipment. As of 2026, one non-machinable stamp costs $1.27: the $0.78 base rate for a one-ounce letter plus a $0.49 non-machinable surcharge.1USPS. First-Class Mail You need one when your envelope is square, rigid, lumpy, or has any exterior fastener like a clasp, button, ribbon, or wax seal.
When You Need One
USPS sorting machines are built for a narrow range of envelope shapes and flexibility. A standard First-Class letter has to fall between 5 and 11½ inches long, 3½ and 6⅛ inches high, and no more than ¼ inch thick.2Postal Explorer. Sizes for Letters Anything outside those dimensions isn’t just non-machinable — it’s a different mail class (flat) with a different rate.
Within those limits, the machines still need an aspect ratio between 1.3 and 2.5 (length divided by height). A square envelope has a ratio of 1.0, so it’s automatically non-machinable no matter how light or thin it is.3About USPS Home. Nonmachinable Criteria That’s the single most common reason people end up needing the surcharge.
The other triggers:
- The envelope doesn’t bend easily. Heavy cardstock, chipboard backing, or a rigid insert like a magnet will fail the flexibility test.3About USPS Home. Nonmachinable Criteria
- The contents create uneven thickness. Pens, pencils, keys, and coins all make an envelope lumpy enough to jam the equipment. Loose keys or coins can make the piece unmailable in a paper envelope entirely, not just non-machinable.4Postal Explorer. 201 Quick Service Guide
- The exterior has clasps, strings, buttons, or similar closures that can catch in the sorter.4Postal Explorer. 201 Quick Service Guide
- The surface isn’t paper. Plastic or foil wrapping counts as non-machinable.4Postal Explorer. 201 Quick Service Guide
What the Postage Costs
The non-machinable surcharge is $0.49, added on top of the regular postage for the letter’s weight.5Postal Explorer. Notice 123 Price List Each additional ounce beyond the first adds $0.29. At each weight tier for a non-machinable letter:
- 1 oz: $0.78 + $0.49 = $1.27
- 2 oz: $1.07 + $0.49 = $1.56
- 3 oz: $1.36 + $0.49 = $1.85
- 3.5 oz: $1.65 + $0.49 = $2.14
First-Class letters top out at 3.5 ounces.1USPS. First-Class Mail Anything heavier moves to large envelope (flat) pricing.
The current dedicated non-machinable stamp features a Luna Moth and is priced at $1.27, so one stamp covers a one-ounce non-machinable letter with no math. For heavier pieces, add a $0.29 additional-ounce stamp for each ounce beyond the first. Many people still call any non-machinable stamp a “butterfly stamp” because the 2010 design used a butterfly silhouette that greeting card makers printed on envelopes needing the surcharge.6United States Postal Service. The Butterfly Stamp The dedicated stamp is a convenience, not a requirement — a Forever stamp plus enough additional postage to reach the total works the same way.
Wedding Invitations and Greeting Cards
This is where most people run into the surcharge. Wedding suites hit nearly every trigger at once: square envelopes, heavy cardstock, an inner envelope, an RSVP card, sometimes a wax seal or ribbon.
Square envelopes always require the surcharge, whatever they weigh. Wax seals make the envelope rigid and uneven, which triggers it too. USPS recommends placing wax-sealed envelopes inside a second, outer mailing envelope so the seal doesn’t crack in processing.7USPS. How to Send a Letter or Postcard Ribbons, twine, and string on the exterior create the same problem as clasps; USPS mail standards explicitly warn against twine or cord because it can interfere with equipment.8USPS Domestic Mail Manual (DMM 300). 601 Mailability
Weigh a finished, fully assembled invitation before you buy postage in bulk. Most wedding suites with an inner envelope, RSVP card, and details insert will cross the one-ounce line, so you’ll need additional-ounce stamps on top of the surcharge.
What Happens If You Skip the Surcharge
Unlike most shortpaid First-Class Mail, which is delivered with “postage due,” a non-machinable letter without the surcharge is returned to the sender.9Postal Explorer. 604 Postage Payment Methods and Refunds Your recipient won’t get the letter and settle the difference at the mailbox. The envelope comes back to you, and you start over.
For time-sensitive mail — invitations, RSVPs, holiday cards — that round trip can add a week or more. A piece with no return address may never reach anyone at all. The $0.49 is cheap insurance.
Where to Buy Them
The most reliable option is the post office counter. A clerk can weigh the piece, confirm it’s non-machinable, and sell you the exact postage. That’s worth doing before you buy 200 stamps for an invitation batch.
You can also order Luna Moth non-machinable stamps online from the USPS Postal Store; they come in sheets of 20 for $25.40. Self-service kiosks inside post office lobbies will print a postage label for the correct amount as well, though you get a plain printed label rather than the moth design.
A tip once you have the postage on: hand non-machinable mail directly to a clerk at the counter instead of dropping it in a collection box. Mail in a blue box will usually be processed correctly, but no one is there to catch a mistake before it comes back to you.
International Letters
The surcharge applies to First-Class Mail International too, at the same $0.49 per letter.10USPS Postal Explorer. Notice 123 Price List Effective January 18, 2026 The physical triggers — square shape, rigidity, clasps, uneven thickness — are the same in practice, though international letters keep the same 3.5-ounce weight ceiling.11Postal Explorer. Country Price Groups and Weight Limits One warning: if a non-machinable international letter is also rigid, non-rectangular, or unevenly thick and exceeds letter dimensions, it moves to package pricing rather than large-envelope pricing, which is a much bigger jump than the domestic equivalent. Weigh and measure carefully before you affix postage.