Clasp envelopes are one of the most useful items in any office supply cabinet. They hold documents flat, they open and close dozens of times without breaking down, and they come in sizes that cover everything from a folded permission slip to a full architectural drawing. The problem is that almost nobody learns how to mail them correctly the first time. The metal clasp that makes them so convenient for interoffice routing is exactly what creates problems when you put one in the mail without preparation.
This guide walks through every step required to mail a clasp envelope through USPS correctly in 2026. Whether you are looking for how to mail clasp envelope formats of any size, need to know how to address clasp envelopes for the first time, or want to understand when clasp envelopes how to mail them becomes a postage calculation question, every answer is here. That includes how to handle the clasp, how to seal the envelope, how to calculate the right postage, how to address the envelope using USPS formatting rules, and where to drop it off. A size-by-size postage table is included at the end, along with answers to the most common questions about clasp envelope mailing. Business Envelopes has been supplying clasp envelopes to US businesses since 1997. Every piece of practical advice in this guide comes from direct experience with how these envelopes behave in real mailing programs.
What Makes Clasp Envelopes Different from Mail
Before getting into the steps, it helps to understand exactly why clasp envelopes require more preparation than a standard letter envelope. The answer comes directly from USPS Domestic Mail Manual section 201.2.1, which defines the characteristics that make a mailpiece non-machinable.
According to USPS Publication 25, a letter-size piece is non-machinable if it has clasps, strings, buttons, or similar closure devices. The metal clasp on a standard brown kraft clasp envelope falls squarely in this category. When a clasp envelope goes through USPS automated sorting equipment with the clasp still attached and unsecured, the metal prongs can catch on conveyor belts, damage adjacent mail, jam rollers, and cause the envelope itself to tear open during transit.
The consequence is a $0.49 non-machinable surcharge on top of whatever flat-rate postage the envelope requires. In some cases, the piece is returned to the sender entirely. The good news is that the fix is clear and takes less than a minute per envelope.
Step 1: Handle the Metal Clasp Before Mailing
You have three options for handling the clasp. The right one depends on whether you plan to reuse the envelope after mailing.
Remove the clasp entirely (best option)
Straighten the metal prongs completely until they point straight out. Then pull the prongs back through the eyelet hole in the envelope body. The clasp comes out cleanly in most cases, leaving a small eyelet hole in the flap. Discard the clasp. The envelope is now a standard flat-document mailer with a gummed flap as its only closure, and the non-machinable classification no longer applies. This is the most reliable method.
Tape the clasp flat (if removal is not possible)
Straighten the metal prongs as flat as possible against the back of the envelope. Apply heavy-duty packing tape over the prongs, covering all metal edges completely. The tape must be firm, adhere to all sides, and must not leave any exposed metal. This reduces the mechanical risk during sorting, but note that USPS technically classifies taped clasps differently depending on the inspector. Removing the clasp is always the cleaner approach.
Pay the non-machinable surcharge (if keeping the clasp matters)
If the envelope will be returned and reused after delivery, leaving the clasp in place is acceptable. Pay the $0.49 non-machinable surcharge at the Post Office counter when you drop off the piece. The postal clerk will assess it and calculate the correct total postage. This works for occasional one-off mailings where reusability is important.
Step 2: Seal the Envelope Correctly
Every clasp envelope from Business Envelopes includes a gummed flap in addition to the metal clasp. The gummed flap is the correct sealing method for mailing. The clasp is for repeated opening and closing during storage or routing. Do not rely on the clasp as the primary seal for a mailed piece.
How to seal the gummed flap
Insert all documents, verify that nothing is protruding past the edges of the envelope, and fold the gummed flap down. Moisten the gummed strip either with a damp sponge, a wet cloth, or the edge of a water source. Do not over-saturate. Press the flap firmly down and hold for several seconds. The seal sets quickly. For high-volume mailing programs, a desktop moistener or water-activated tape dispenser makes this faster and more consistent.
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"These clasp envelopes are perfect. Never ripped or broken. I will reorder again."
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Step 3: Calculate Clasp Envelope Postage Correctly
This is where most people make mistakes. Clasp envelopes are not letters. Every standard clasp envelope size (6x9, 9x12, 9.5x12.5, 10x13, and 10x15) qualifies as a large envelope, also called a flat, by USPS. Flat-rate postage applies, not letter rate. A single Forever stamp ($0.73 in 2026) is not enough postage for any clasp envelope.
USPS 2026 postage rates for clasp envelopes
| Envelope Size | USPS Classification | First Ounce (Clasp Removed) | First Ounce (Clasp Attached) | Each Additional Ounce |
|---|---|---|---|---|
| 6 x 9 | Large envelope (flat) | $1.63 | $2.12 (includes $0.49 surcharge) | $0.22 |
| 9 x 12 | Large envelope (flat) | $1.63 | $2.12 (includes $0.49 surcharge) | $0.22 |
| 9.5 x 12.5 | Large envelope (flat) | $1.63 | $2.12 (includes $0.49 surcharge) | $0.22 |
| 10 x 13 | Large envelope (flat) | $1.63 | $2.12 (includes $0.49 surcharge) | $0.22 |
| 10 x 15 | Large envelope (flat) | $1.63 | $2.12 (includes $0.49 surcharge) | $0.22 |
Rates based on USPS Notice 123, effective January 2026. All clasp envelope sizes qualify as flats at the same base rate. Postage increases with weight in $0.22 increments per ounce above the first. The maximum weight for First-Class flats is 13 ounces. Heavier pieces move to Priority Mail.
How to weigh a clasp envelope
The envelope itself adds weight. A standard 9x12 brown kraft clasp envelope weighs approximately 0.7 to 0.9 ounces before any contents. A 10x13 weighs approximately 1.0 to 1.2 ounces. Add content weight to determine total weight and the correct postage tier. Any fraction of an ounce rounds up to the next full ounce for billing. A piece weighing 2.1 ounces bills at the 3-ounce rate.
The flexibility test
USPS also classifies flats by flexibility. A large envelope must be able to bend at least one inch when extended halfway off a flat surface. An overstuffed clasp envelope with rigid contents that cannot flex will be reclassified at the Post Office counter, potentially as a package, with significantly higher postage. Keep contents flat and uniform in thickness.
Step 4: Address the Clasp Envelope Correctly
Addressing rules for clasp envelopes are the same as for any large flat mailer. USPS Publication 28 defines the standards for machine-readable addresses. These apply regardless of envelope size or type.
Delivery address placement
Place the delivery address in the center-lower portion of the envelope face. On a 9x12 envelope, center the address block horizontally and position it in the lower half of the face. Use all capital letters throughout. Include no punctuation except the hyphen in a ZIP+4 code. Write the suite number on the same line as the street address, never on a separate line below it. USPS OCR systems read addresses from the bottom up, so a suite on a separate line can cause misrouting.
Correct address format example
BUSINESS ENVELOPES
308 MUSEUM VILLAGE RD
MONROE NY 10950-1234
JANE SMITH
ACME CORPORATION
1234 MAIN ST STE 500
CHICAGO IL 60601-1234
Return address
Place your return address in the upper-left corner of the face, at least one quarter inch from the left and top edges. A return address is essential for clasp envelopes. If the piece cannot be delivered for any reason, it goes to the USPS Mail Recovery Center if there is no return address, rather than back to you.
The barcode clear zone
Keep the bottom 5/8 inch across the entire face of the envelope completely free of any text, printing, labels, or design elements. USPS applies a delivery point barcode in this strip during automated sorting. Any content in this zone prevents barcode placement and removes the piece from automated processing, adding transit time and risk.
Postage placement
The stamp or postage meter imprint goes in the upper-right corner. Nothing else should appear in this corner. On a large clasp envelope, applying the correct flat-rate postage requires either a Post Office counter transaction, a postage meter, or online postage printed from USPS.com or an approved vendor.
Step 5: Choose How to Drop Off the Mail
Once the envelope is sealed, addressed, and has the correct postage, you have several options for submitting it to USPS.
Blue collection boxes
Standard USPS blue collection boxes accept large envelopes, but there are two conditions. First, the envelope must fit through the slot, and most 9x12 envelopes fit flat, but a 10x15 may not. Second, pieces weighing more than 10 ounces with stamp postage cannot go in a collection box. They must go to a Post Office counter. If your sealed clasp envelope weighs more than 10 ounces, take it directly to the Post Office.
Post Office counter
The counter is the most reliable drop-off point for clasp envelopes, especially if the clasp is still attached and you are paying the non-machinable surcharge. The clerk will weigh the piece, assess it for any surcharges, and apply the correct postage. This eliminates any risk of underpayment.
Carrier pickup
You can request a free carrier pickup from USPS.com for outgoing packages and large envelopes. This is practical for businesses mailing multiple clasp envelopes on the same day. Schedule by noon for same-day pickup in most areas.
Size-by-Size Postage Guide for Clasp Envelopes (2026)
Every clasp envelope size at Business Envelopes uses the same USPS flat-rate postage structure. The table below gives the postage for common weight tiers.
| Weight | Flat Rate (Clasp Removed) | With Non-Machinable Surcharge | Typical Contents |
|---|---|---|---|
| 1 oz or less | $1.63 | $2.12 | 1-2 pages, single document |
| 2 oz | $1.85 | $2.34 | 4-6 pages, light packet |
| 3 oz | $2.07 | $2.56 | 8-10 pages, report packet |
| 4 oz | $2.29 | $2.78 | 12-15 pages, multi-page form |
| 5 oz | $2.51 | $3.00 | 15-20 pages, thick contract |
| Over 13 oz | Priority Mail pricing applies and is taken to the Post Office counter | ||
These rates apply to all clasp envelope sizes from 6x9 through 10x15. The size of the envelope does not change the postage rate, and only the weight of the sealed piece determines the tier. Always weigh after sealing.
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Verified buyer | 10x13 Brown Clasp Envelopes
When to Use a Different Envelope Instead of a Clasp Envelope
Clasp envelopes are excellent for interoffice routing, document storage, and occasional mailing. But for regular high-volume mailing programs, a different format often makes more practical and economic sense.
Use a peel-and-seal catalogue envelope for frequent mailing
If you mail flat documents regularly and do not need to reuse the envelope after delivery, a peel-and-seal catalogue envelope of the same size eliminates the clasp preparation step. No clasp to remove or tape, no non-machinable surcharge, faster sealing, and the same document protection in transit. See the full range at self seal envelopes.
Use Tyvek for high-value or moisture-sensitive documents
For documents that must arrive in perfect condition regardless of weather or handling, Tyvek envelopes offer tear resistance and moisture resistance that kraft paper cannot match. They mail at the same flat rate and eliminate both the clasp issue and the paper integrity concern. Available in Tyvek envelopes.
The two-envelope approach
Many offices use clasp envelopes for the routing stage and switch to a plain catalogue envelope for the final mailing. The document travels through internal review in the clasp envelope, then gets transferred to a gummed-flap or peel-and-seal envelope of the same size for USPS delivery. The clasp envelope goes back into rotation for the next document cycle. This is the most cost-efficient approach for teams that value the clasp envelope's reusability for internal routing while avoiding surcharges on outbound mail.
For all available sizes in both clasp and non-clasp flat formats, see all sizes of envelopes.
For more background on whether USPS accepts clasp envelopes and the regulations that apply, see the full guide at can you mail clasp envelopes.