Clasp envelopes are one of the most misunderstood products in any office supply catalogue. People order them for mailing and are surprised when the post office adds a surcharge. People reach for them because the clasp feels more secure, without knowing that the same clasp changes how USPS classifies the piece. And people overlook them for interoffice routing, which is the one use case where a clasp envelope is irreplaceable.

This guide covers how the metal clasp mechanism works, all five sizes and what fits in each, and the USPS non-machinable surcharge that applies to every piece in every mail class. It also covers the dual closure system that makes this format the right choice for documents that circulate before mailing, how to mail and address a clasp envelope correctly, and which industries rely on it as their default rather than a specialty item.

What a clasp envelope actually is

A clasp envelope is a heavy-duty kraft paper envelope with two closure mechanisms built into the same flap: a metal two-pronged clasp and a gummed adhesive strip. The clasp provides a mechanical, reopenable closure. The gum strip provides a permanent adhesive seal for final mailing. The two work independently, which is the entire point.

The metal clasp is a flat, two-pronged brad fastener, identical in concept to the brass fasteners used in school folders and three-hole report covers. The prongs pass through a reinforced hole in the envelope body. You bend the prongs outward to lock the flap down, and you straighten them again to reopen. This mechanism can be cycled 20 to 30 times before the metal fatigues, which makes this envelope type the standard for any document that passes through multiple hands before final mailing or filing.

The gum strip is the moistenable adhesive on the inside edge of the flap, identical to the closure on a standard brown kraft catalogue envelope. When you are ready to mail permanently, you moisten the gum strip and press the flap down alongside the clasp. The result is a piece with both mechanical and adhesive closure, which is stronger than either alone.

Most offices use the clasp for routing and storage, then activate the gum strip when the document is ready for final mailing. Some operations use the clasp only, which works for internal routing but does not produce a sealed mailpiece as USPS defines it.

The anatomy of a clasp envelope

Understanding the named parts makes it easier to specify the right format and diagnose problems when closures fail. The envelope body is the main paper structure, manufactured from 28 lb brown kraft in the formats Business Envelopes carries. The flap is the top section that folds over the opening to seal it. The flap can be closed using the metal clasp, the gum strip, or both.

The metal clasp is the two-pronged fastener positioned near the opening edge of the flap. It consists of a metal brad with two flat prongs. The prongs pass through the eyelet hole in the envelope body and bend outward to lock the flap closed. Straightening the prongs releases the flap.

The reinforced eyelet is the hole in the envelope body through which the clasp prongs pass. Quality clasp envelopes reinforce this eyelet with a metal grommet that prevents tearing under repeated opening and closing cycles. Lower-quality envelopes use a plain punched paper hole that can tear under sustained use or heavy contents.

The gum strip is the moistenable adhesive on the inside edge of the flap for permanent sealing.

Other names for clasp envelopes

These envelopes go by several names depending on the industry and context. A prong envelope refers specifically to the two-pronged brad mechanism that locks the flap. Expansion envelope is used when referring to formats with gusset sides, though most standard versions do not have gussets. Manila envelope is a broader term for any large brown kraft flat-document envelope, with or without a clasp. Catalogue envelope refers to the open-end or open-side large format without a clasp. Knowing these alternative names helps when searching supplier catalogues or specifying formats to vendors who may use different terminology.

How to use a clasp envelope: step by step

This sequence applies to any clasp envelope, regardless of size.

Step one: Insert the contents. Place documents inside the envelope. Ensure the contents fit without the stack pushing against the flap when it closes. If the contents push the flap away from the body, size up before proceeding.

Step two: Fold the flap over the envelope body. Align the flap cleanly with the body surface so it lies flat.

Step three: Secure the clasp. Insert the two metal prongs through the eyelet hole in the envelope body. Bend each prong outward in opposite directions until they lie flat against the back of the envelope body.

Step four: Address the envelope correctly. The delivery address goes in the center of the envelope face. Use all capital letters, no punctuation, and black or blue ink only. Red ink is not acceptable for USPS automated scanning. A correctly formatted address looks like this:

JANE SMITH ACCOUNTING DEPARTMENT 123 MAIN ST APT 4B NEW YORK NY 10001-1234 Include the recipient's name on the first line, the company or department name on the second line if applicable, the street address with the apartment or unit number on the same line, and the city, state, and ZIP code on the final line. Use the ZIP+4 code when available, since it speeds processing and reduces the chance of misdelivery. If you are mailing to a specific person at a company, add ATTN: PERSON NAME on the first line above the company name.

Your return address goes in the upper-left corner in the same all-capitals format. If the piece cannot be delivered and has no return address, USPS sends it to the Mail Recovery Center rather than back to you. Include it on every piece. Keep the bottom 0.625 inches of the envelope face completely clear. USPS automated equipment prints a Delivery Point Barcode in that zone after reading the delivery address.

Step five: Apply postage and mail. A clasp envelope requires the $0.49 non-machinable surcharge in addition to standard postage because the metal clasp is classified as a protrusion under USPS DMM 201.3.8. To mail without the surcharge, remove the clasp before mailing and seal with the gum strip only.

How to mail a clasp envelope: the complete process

How to mail a clasp envelope correctly involves more than applying a stamp. Several decisions at the mailing stage determine whether your piece reaches its destination at the expected cost.

First, decide whether the clasp will stay or go. Mailing a clasp envelope with the clasp still engaged triggers the $0.49 non-machinable surcharge automatically. The USPS classifies any piece with a clasp, string, or button closure as non-machinable and requires hand cancellation. You have three options.

Option one: Remove the clasp before mailing. Straighten the prongs fully, pull them back through the eyelet, and discard. Seal with the gum strip. The piece mails at the standard flat rate without the surcharge.

Option two: Place the clasp envelope inside a larger plain envelope. If the clasp envelope must stay intact for reuse or routing markings, slide it inside a larger gum-seal or peel-and-seal catalogue envelope of the next size up. The outer envelope carries no clasp, mails cleanly at flat rate, and the clasp envelope inside is protected on arrival.

Option three: Pay the surcharge. If removing the clasp is not practical, budget $0.49 additional per piece and mail as non-machinable.

Second, weigh the sealed piece before buying postage. A common mistake when learning how to send a manila envelope in the mail is measuring the envelope while the flap is open or before the contents are inserted. Always weigh with the flap sealed and all contents inside.

Third, understand what happens with insufficient postage. If a clasp envelope goes out with too little postage, USPS returns it to the sender marked postage due. Alternatively, it is delivered to the recipient who must pay the shortfall plus a handling fee before receiving the piece. Weigh before you seal and calculate correctly before mailing.

Fourth, check size classification. The 6x9 clasp envelope can sometimes qualify for the lower First-Class letter rate rather than the flat rate if contents are flexible and thin. A 6x9 measures 6 inches tall, within the 6.125-inch letter-rate height maximum. If the sealed piece is under 0.25 inches thick and the contents are flexible, it may qualify as a letter. The 9x12 and larger sizes are always classified as flats.

How the dual closure system changes the way documents move

Standard gummed envelopes are one-way instruments. Once sealed, opening them destroys the flap. That is appropriate for outgoing mail, but impractical for any document that needs review, signature, or additional materials before it goes out.

This format breaks that one-way constraint. A purchase order that needs three approvals before mailing travels in a clasp envelope. Reviewer one opens the clasp, signs, refastens the clasp, and passes it to reviewer two. Reviewer two does the same. When the process is complete, the final person activates the gum seal for mailing. Nobody has used tape. Nobody has used a new envelope. The clasp envelope has performed three distinct functions: carrier, document holder, and final mailpiece.

This is why legal offices, school administration offices, government agencies, and healthcare facilities standardize on these envelopes for document routing and mailpiece functions, rather than rubber banding loose documents together or using file folders that cannot be mailed.

The USPS mailing rule every clasp envelope buyer needs to know

USPS Domestic Mail Manual Section 201.3.8 explicitly states that letter-size pieces with clasps, strings, buttons, or similar closure devices are classified as non-machinable. USPS Quick Service Guide 201a confirms it directly: clasps, strings, staples, buttons, or protrusions that might impede or damage mail processing equipment are prohibited from automation-compatible processing. USPS hand-cancels these pieces and processes them separately from automated mail.

What this means in practice: every piece you mail through USPS with the clasp attached, at any size, at any weight, is subject to the non-machinable surcharge. The surcharge is currently $0.49 per piece and applies regardless of whether the envelope falls within letter dimensions, flat dimensions, or anything in between.

What this means for your mailing costs

A 9x12 clasp envelope already mails as a flat at the first-class flat rate of $1.63. The non-machinable surcharge adds $0.49 on top. The total minimum postage for a 9x12 clasp envelope at one ounce is $2.12 rather than $1.63. On a batch of 100 pieces, that is $49 in additional postage compared to a gum-seal-only 9x12 envelope of the same size.

Clasp vs string-and-button closure: the difference

String-and-button envelopes use a different closure mechanism. Two small circular discs are mounted on the envelope, one on the flap and one on the body. A string wrapped around both discs in a figure-eight pattern holds the flap closed. Unlike a metal clasp, string-and-button closures create no protrusion above the paper surface, which means they do not automatically trigger the USPS non-machinable surcharge. A loosely wound string can unwind under vibration or jostling in a mail tray, making the clasp more secure for repeated routing use.

All five sizes at Business Envelopes: what fits in each

Business Envelopes carries clasp envelopes in five standard sizes, all in heavy-duty 28 lb brown kraft with a metal clasp and a gum flap dual closure.

6 x 9 inches: The compact format. Holds a half-folded 8.5 x 11 sheet, small multi-page packets, 5x7 photographs, and report cards. Can qualify for letter rate if contents are flexible and under 0.25 inches thick.

9 x 12 inches: The most widely ordered size. Holds a standard 8.5 x 11 sheet completely flat, multi-page reports, certificates, and diplomas. Always mails as a flat.

9.5 x 12.5 inches: The half-inch additional clearance accommodates tabbed dividers, report covers, and plastic sleeves that push against 9x12 walls.

10 x 13 inches: For thicker document packets, legal-size 8.5 x 14 sheets mailed flat, and content stacks that create seam stress in a 9x12.

10 x 15 inches: The largest format. For oversized documents, multi-page bound reports, large certificates, and folded posters.Clasp vs gum-seal-only catalogue envelopes: how to choose

Use clasp envelopes when the document travels through multiple people, requires sequential review, may need additional inserts before mailing, or functions as a reusable interoffice routing container.

The clasp reusability factor

Most clasp envelopes support 20 to 30 open-and-close cycles before the metal shows visible fatigue. When the clasp fatigues before the envelope body, remove the spent clasp and use the envelope with a gum-seal closure only for the remaining life of the paper body.

Custom printing on clasp envelopes

Custom printing is available on clasp formats for organizations that want their logo, return address, and brand elements on the outer face. Business Envelopes offers custom printing with a PDF proof process and no minimum order. Available in printed envelopes.

Industries and use cases where clasp envelopes are the default

Legal offices: Contracts, disclosure documents, and court filing packages move through multiple reviewers. The clasp handles routing. Many legal offices maintain dedicated clasp envelope stock for internal circulation entirely separate from their outgoing mailing stock.

School administration: Report cards and permission slip bundles go home and return in clasp envelopes. Elementary schools are among the highest-volume buyers because this workflow is a daily routine.

Healthcare administration: Patient charts, referral packets, and medical records releases require a container that can be opened and resealed at each stage of the process.

Government and public agencies: Permit applications, regulatory filings, and licensing documents involve multi-step review workflows.

Accounting and financial services: Monthly receipt collection, expense report routing, and client document submissions. One Business Envelopes customer uses 10x13 clasp envelopes for weekly receipt scanning across an employee team, noting they have never ripped or broken.

What no clasp envelope guide tells you

The gum strip degrades before the clasp in humid storage.

The clasp still functions perfectly, but the gum adhesive weakens. Test the gum strip on a sample before relying on it for final mailing in humid conditions.

The reinforced eyelet matters more than buyers realize. Lower-quality envelopes use a plain punched paper hole that tears under sustained use or heavy contents. Business Envelopes uses reinforced eyelet construction throughout.

These envelopes work as file folders. The clasp mechanism is identical to brad fasteners in hanging file folders. Offices use them in file cabinet drawers with the added benefit of a closeable flap.

The size-up logic is more urgent with clasp envelopes than gum-seal. An overstuffed clasp cannot close the prongs fully because the document stack prevents the flap from seating flat. Size up before overstuffing rather than after.

Rigid contents can push the piece toward parcel pricing. Very thick or rigid stacks inside a clasp envelope may be reclassified by USPS from flat to parcel, which carries significantly higher postage rates. When contents are thick and rigid, weigh and present the sealed piece at a USPS counter before mailing a large batch.

Clasp vs Tyvek: when to use each

Use clasp envelopes when the document needs to circulate before mailing and routing access matters more than moisture protection. Use Tyvek envelopes when the document cannot arrive torn or wet, and routing access is not required. For documents needing both, use the clasp format for routing, then transfer into Tyvek for final mailing.

Postage and mailing checklist for clasp envelopes

Postage and mailing checklist for clasp envelopes

Check one: Decide whether the clasp stays or goes before sealing.

Check two: Insert all contents before sealing. Weigh the sealed envelope on a postal scale.

Check three: Calculate postage from the sealed weight. For flat-rate pieces: $1.63 for the first ounce, $0.29 per additional ounce, plus $0.49 if the clasp is present.

Check four: Confirm delivery and return addresses are in black or blue ink, all-capital letters, no punctuation.

Check five: Keep the bottom 0.625 inches of the face clear of all labels.

Check six: For 6x9 clasp envelopes, verify whether the sealed piece qualifies for letter rate. If under 6.125 inches tall, under 11.5 inches long, and under 0.25 inches thick, it may qualify for letter postage rather than flat rate.

Why choose Business Envelopes for clasp envelopes

Business Envelopes has supplied clasp envelope stock to US offices, legal practices, schools, healthcare facilities, and government agencies since 1997. All five sizes from one catalogue in 28 lb heavy-duty brown kraft with dual gum-and-clasp closure. Reinforced eyelet construction. Custom printing available with PDF proof before production. Free shipping, no minimums. Blank stock ships the same or next business day.

The bottom line

Clasp envelopes exist because documents sometimes need to travel through multiple hands before going into the mail. The clasp handles routing. The gum strip handles the final seal. Know the $0.49 USPS non-machinable surcharge before budgeting postage. Remove the clasp before mailing to eliminate it. Size up before overstuffing. And always address in all-caps with black or blue ink, no punctuation, keeping the bottom 0.625 inches clear for USPS barcode printing.