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How to Mail a Letter: The Complete Step-by-Step Guide
Mailing a letter sounds simple until something goes wrong. A returned invoice because the address was off-center. A contract that came back stamped "insufficient postage." A greeting card that triggered a non-machinable surcharge nobody expected. This guide covers every step from choosing the right envelope to getting the piece into USPS hands, with the specific details most guides skip.
What you need before you start
Before you seal anything, gather four things: your letter or document, the correct envelope for it, your recipient's complete address, and the right postage. Getting any one of these wrong costs you time and money on the return trip.
The most common envelope for standard business mail is the #10 envelope (4.125 x 9.5 inches). It holds one 8.5 x 11 inch sheet folded in thirds and mails at the standard First-Class letter rate. If you are sending something that should not be folded, a 9x12 catalog envelope holds it flat. If you need the recipient address to show through the envelope face without double printing, a window envelope is the right choice for invoices and statements.
Step 1: Choose the right envelope
Most people grab whatever envelope is nearby. That is the first mistake. USPS processes letters through high-speed optical character recognition (OCR) machines. The envelope has to meet specific physical requirements or it gets kicked out, charged a surcharge, or returned.
Here are the USPS physical standards for a letter to qualify for the First-Class letter rate:
|
Dimension |
Minimum |
Maximum |
|
Height |
3.5 inches |
6.125 inches |
|
Length |
5 inches |
11.5 inches |
|
Thickness |
0.007 inch |
0.25 inch |
A standard #10 envelope at 4.125 x 9.5 inches sits comfortably within those limits. If your envelope falls outside them, even by a little, it stops being a letter and becomes either a flat or a nonmachinable piece, both of which cost more to mail.
What triggers a non-machinable surcharge (an extra 49 cents per piece as of 2025):
Square envelopes trigger it automatically because their aspect ratio does not fall within the 1.3 to 2.5 range USPS requires. Envelopes with clasps, strings, buttons, or rigid contents trigger it. Envelopes that are lumpy or cannot bend through sorting equipment trigger it. This is a detail most guides leave out entirely: even a standard-sized envelope with something rigid inside, like a flash drive or a key, becomes non-machinable. The object inside changes the rate, not just the envelope.
Choosing by what you are mailing:
For a single folded letter or invoice, use a #10 envelope. For invoices where the address should show through, use a window envelope and make sure your letterhead address block aligns with the window position before ordering in bulk. For legal documents or contracts that must arrive flat, use a 9x12 catalog envelope. For documents that genuinely cannot arrive damaged or torn, use a Tyvek envelope made from DuPont polyethylene fiber that resists tearing and moisture from the material itself, not from a coating.
For high-volume mailings like weekly invoice runs, self seal envelopes eliminate the need to lick or moisten every flap. The adhesive strip stays protected until you peel the liner. On a run of 500 envelopes, that is a meaningful time saving.
Step 2: Write or print your letter
Typed letters are the standard for business correspondence. Use 10, 12, or 14 point type for the body. Smaller fonts can be hard for the recipient to read and do not scan well if the letter is ever digitized.
For handwritten personal correspondence, write clearly. Postal staff rely on legible addresses. The body of a handwritten letter has no postal requirements, only the outside of the envelope does.
Fold standard 8.5 x 11 paper into thirds to fit a #10 envelope. The correct fold: bring the bottom third up, then fold the top third down over it. Insert with the top of the letter facing up and toward the envelope flap. This way, when the recipient opens the envelope and removes the letter, the first thing they see is the top of your page, not the fold.
Step 3: Address the envelope correctly
USPS uses optical character recognition to sort mail automatically. The machine reads the address block and routes the piece. If the address is not in the right zone or is formatted incorrectly, it goes to manual processing, which slows delivery and increases the chance of misdelivery.
Return address: Top-left corner. Your full name or company name, street address, city, state abbreviation, and ZIP code.
Recipient address: Center of the envelope. Name on the first line, street address including any unit or suite number on the second line, city, two-letter state abbreviation, and 5-digit ZIP code (or ZIP+4 for better routing accuracy) all on the third line.
USPS recommends all capitals, no punctuation, and left-justified alignment for the best machine readability. In practice, standard title case with proper punctuation processes correctly for most First-Class mail, but for bulk mailings, follow the USPS standard.
Address line format the way USPS recommends it:
JOHN SMITH
ACME CORPORATION
123 MAIN ST STE 400
CHICAGO IL 60601-1234
One thing almost no guide mentions: apartment and suite numbers go on the same line as the street address, not on the line below. A suite number on its own line below the street address causes sorting confusion in some automated systems and can delay delivery.
For window envelopes: The delivery address must show completely through the window with at least an eighth-inch clearance on all sides. Test with your actual letterhead before committing to a large order. If the address block shifts when the letter is folded and inserted, it will not be visible through the window and the piece fails postal standards.
Step 4: Seal the envelope
For standard gum-seal envelopes, moisten the flap and press firmly. For self seal envelopes with a peel-and-press strip, remove the liner and fold the flap down under firm pressure. For clasp envelopes used with larger documents, close the gum flap first, then fold the clasp prongs down over it.
One thing most guides do not tell you: If you are using a security-tinted envelope for checks or sensitive documents, the tinted pattern is on the inside liner. It blocks contents from being read when held to a light source. But the tinting only works if the envelope is sealed properly. A partially sealed flap on a check envelope defeats the security feature entirely. Run your finger firmly across the full length of the flap after closing.
Step 5: Calculate and apply postage
Postage for a letter depends on weight, size, and destination.
|
Letter type |
Postage needed |
|
Standard 1 oz domestic (up to 4 sheets + envelope) |
1 Forever stamp (78 cents) |
|
2 oz domestic |
1 Forever stamp + 1 Additional Ounce stamp ($1.07) |
|
3 oz domestic |
1 Forever stamp + 2 Additional Ounce stamps ($1.36) |
|
Square or non-machinable 1 oz |
1 Forever stamp + non-machinable surcharge ($1.27) |
|
Domestic postcard |
1 Postcard stamp (61 cents) |
|
International 1 oz |
1 Global Forever stamp ($1.70) |
|
Large envelope / flat 1 oz |
$1.63 |
The stamp goes in the top-right corner. One Forever stamp covers one ounce. As a rule of thumb, four sheets of standard 20 lb printer paper plus a #10 envelope equals approximately one ounce. Add a return reply envelope inside and you are likely over one ounce and need an additional ounce stamp.
Weigh before you seal if you are unsure. A kitchen scale works fine. The difference between one ounce and 1.1 ounces is 29 cents extra postage. If you do not add it and the piece is returned, you lose the time and cost of reprinting and remailing.
For businesses running high-volume invoice or statement cycles, the math on stamps adds up quickly. At 78 cents per piece on retail stamps versus 74 cents through a metered postage platform, a business sending 500 letters per week saves $1,040 per year just on postage. For that same volume, switching to self seal envelopes eliminates the labor cost of moistening flaps on every piece.
Step 6: Send your letter
Once the envelope is addressed, sealed, and stamped correctly, you have four options:
Blue USPS collection box: The fastest option for outgoing mail. Drop it in before the posted pickup time. Most boxes have two daily pickups. Times are posted on the box itself.
Your home or office mailbox: Place stamped, sealed mail in your mailbox and raise the red flag. Your letter carrier will take it on their next delivery. Note: carriers are not required to stop at mailboxes that do not have deliveries scheduled that day. If timing matters, use a collection box or post office.
Post office counter: Hand it directly to a postal employee. Required for pieces over 10 ounces if using only stamps. Also the right option if you need a postmark, certified mail receipt, or special services.
Schedule a pickup: USPS offers free package pickup at your address. For letters, this works best for businesses with regular outgoing mail volume.
Envelope types and when to use each one
Most guides cover addressing and postage. Very few explain how the wrong envelope type costs you money or causes delivery failure. This is the part that matters most for businesses mailing at volume.
#10 envelopes: The US standard for all business correspondence. 4.125 x 9.5 inches. Holds a Letter sheet folded in thirds. Available plain, security tinted, window, double window, and in self-seal or peel-and-seal closures.
Window envelopes: The address on your printed letterhead shows through the window cutout. Eliminates the need to type or print addresses on the envelope separately. Used for invoices, statements, and any piece where the recipient address is already on the enclosed document. The window must align correctly with the address block or the piece fails USPS processing standards.
Self seal envelopes: Peel-and-press closure with no moisture required. The adhesive strip is protected by a release liner until you are ready to seal. Available in all major commercial sizes. Stores in normal office conditions without risk of pre-sealing from humidity.
Check envelopes: Security tinted interior pattern that blocks contents from being visible when held to light. Used for payroll checks, sensitive financial documents, and anything that should not be readable through the envelope. Available in standard and double-window formats.
Tyvek envelopes: Made from DuPont high-density polyethylene fiber. Tear-proof, puncture-resistant, and moisture-resistant from the material structure itself, not a surface coating. When a contract, legal filing, architectural drawing, or medical record genuinely cannot arrive damaged, paper is not the right material. Available in 6x9, 9x12, 10x13, and 10x15.
Regular envelopes: Standard gum-seal commercial envelopes for everyday correspondence. The broadest category covering the full range of commercial sizes from #6 3/4 through catalog sizes.
Colored envelopes: Available in multiple colors for events, invitations, and correspondence where visual impact matters. Use the same addressing and postage rules as standard white envelopes.
What makes Business Envelopes different
Business Envelopes has supplied commercial envelopes to US offices, law firms, healthcare facilities, nonprofits, and direct mail operations since 1997. Here is what separates this catalog from a general office supply retailer.
Every format in one place. #10 plain, window, tinted, self seal, peel and seal, double window, colored, printed. Catalog sizes from 6x9 through 10x15. Tyvek. Clasp. Coin. Everything a complete mailing program needs, from one supplier.
Custom printing that ships in 7 to 10 business days. Return address, company logo, and color printing on any envelope in the catalog. A PDF proof is provided before production. Nothing prints without your approval. Artwork stays on file for reorders so you never have to re-upload files.
No minimums. A single box of #10s ships the same day as a pallet order. Free ground shipping on every order to the contiguous 48 states regardless of order size.
Self seal in every standard size. Every major commercial format is available in self seal or peel and seal configuration. No moisture required, no pre-sealing from humidity in storage, no hand fatigue on large runs.
Why the envelope type affects USPS processing
Here is something almost no mailing guide explains in plain terms. USPS sorts mail through machines running at high speed. The machine reads the address, reads the barcode, and routes the piece. For this to work, the envelope has to behave predictably in the machine. Envelopes that are too stiff, too lumpy, wrong aspect ratio, or have exterior clasps jam the equipment or get rejected by the sorter.
When a piece gets rejected by automated sorting, it goes to manual processing. Manual processing is slower and more expensive for USPS to operate. The non-machinable surcharge (49 cents as of 2025) exists specifically to recover that cost. The surcharge is not a penalty for you doing something wrong. It is a cost recovery mechanism that applies any time a piece requires human handling instead of machine handling.
Understanding this explains why certain envelope choices cost more to mail:
Square envelopes have an aspect ratio of 1:1, which falls outside the 1.3 to 2.5 range machines require. They cannot be sorted mechanically without being reoriented.
Envelopes with exterior clasps cannot pass through standard sorting channels without catching or jamming.
Envelopes containing rigid objects like USB drives, coins, or hard plastic cards create a thickness variation that machines cannot process.
For most business mail, using a standard #10 envelope with properly folded contents eliminates all of these issues. The #10 was designed around the requirements of postal processing equipment. Every measurement is optimized for machine handling.
The bottom line on mailing a letter
The envelope is not an afterthought. Every step in this process, from the physical size of the envelope to how the address block is positioned, affects whether your letter gets to its recipient on time, at the right cost, and in good condition. The most common and expensive mistake businesses make is treating envelopes as a commodity and grabbing whatever is cheapest. A non-machinable surcharge on 500 letters per week is $12,740 in unnecessary postage over a year. A window envelope with a misaligned address block means reprinting and re-mailing an entire invoice run. Getting these details right upfront is cheaper than fixing them after.