In direct mail marketing, the format is the physical form of the mailpiece: a postcard, a letter in an envelope, a folded self-mailer, a brochure, a catalog, a dimensional piece. It decides how much room you have for the message, how the piece feels in the hand, and - through the USPS size and weight categories - what the postage costs. Choosing among the main mailing formats is one of the first real decisions in any direct mail campaign, because everything downstream, from artwork to budget, follows from it.
Unlike a digital ad that scrolls past, physical mail lands in the hand and stays on the counter, which is a large part of why direct mail advertising still earns strong response rates. But that advantage only pays off when the format fits the job. Here are the types of direct mail that actually get mailed, what each is good for, who uses it, and the rules that govern them.
The one distinction that drives cost: letters vs. flats
Before the formats themselves, one USPS distinction sits underneath all of them. The United States Postal Service prices mail by *category*, and the two that matter for most direct mail are letters and flats.
A letter fits within 6-1/8 inches high, 11-1/2 inches long, and 1/4 inch thick. A piece larger than any of those, up to 12 by 15 by 3/4 inch, is a flat. Flats cost meaningfully more to mail than letters. This is why so much direct mail is designed to stay just inside letter size: a postcard, self-mailer, or folded brochure that crosses into flat territory can nearly double its postage without adding a matching lift in response.
So the practical question behind every format choice is: does this need to be a flat, or can it do its job as a letter? Keeping a piece within letter size is one of the most cost effective decisions in the whole campaign, and it is decided by the format long before the artwork is finished.
Postcards
A postcard is the simplest addressed mailpiece: a single card, no envelope, message on both sides. Because there is nothing to open, the message is seen the moment it is pulled from the mailbox, which is why direct mail postcards lead in raw exposure and remain the workhorse of direct mail.
Two size classes matter. A standard postcard stays within letter size and mails at the lowest prices. An oversized postcard - commonly 6 by 9 or 6 by 11 inches - is still a letter for USPS purposes as long as it stays within the letter maximums, so it buys more space and standout without jumping to flat postage. Push past 6-1/8 by 11-1/2 inches and it becomes a flat.
Postcards suit short messages built around a single clear offer: an event, a reminder, a sale, a grand opening, or a limited-time discount. They are a natural fit for local businesses, real estate agents, and healthcare providers who want frequent, low-cost touches with a target audience. Heavier stocks such as 14pt and 16pt cover give the piece a premium feel without changing the postage category, and a QR code turns the card into a bridge to a landing page or booking form.
Letters in envelopes
A letter in an envelope is the format for messages that need to feel personal, formal, or confidential: fundraising appeals, statements, renewal notices, official and constituent mail, and any offer that includes a reply device. The outer envelope creates a small moment of opening, and what is inside can be personalized page by page with variable data printing.
Most business letter mail uses a #10 envelope (the standard business size) or a 6 by 9 inch envelope, both of which keep the piece in the letter category. The format also supports reply envelopes and letterhead with variable data, so a single mailing can carry a personalized letter, a response form, and a branded business reply envelope together - a full letter package assembled and inserted as one piece.
Personalized direct mail in an envelope is where one-to-one relevance pays off: using variable data printing to change the name, the offer, and even the imagery per recipient consistently lifts response rates over a generic piece. The trade-off is cost and effort - envelopes, inserting, and personalization make this the most involved format to produce - so it earns its place with existing customers and high-value segments where the reply device or the personalization is doing real work.
Folded self-mailers
A folded self-mailer travels through the mail without an envelope, but unlike a postcard it folds, so it offers multiple panels of space. A folded self-mailer is the middle ground between a postcard and an enveloped letter: more room than a postcard for detailed information, less cost and production than an envelope mailer.
The catch is closure. USPS requires the open edge of a folded self-mailer to be sealed with tabs (wafer seals) so it does not spring open and jam automated equipment. The number and placement of tabs depend on the fold and the final size, and getting them wrong is one of the most common reasons a self-mailer is rejected or surcharged. Fold orientation, final dimensions, and tab placement all have to meet USPS standards - which is exactly the kind of thing to confirm before printing, not after.
Flyers and brochures
Flyers and brochures are folded self-mailers with a marketing job. A flyer or brochure that folds - a tri-fold or half-fold - gives you distinct panels to structure a story: a cover to earn attention, inside panels to make the case with high quality images and detailed information, a back panel for the call to action. A well designed brochure is a strong choice when one product or service needs more explanation than a postcard allows.
The same tabbing and size rules as any self-mailer apply. Keep the folded piece within letter size to hold letter postage; a large unfolded brochure that only folds down to flat dimensions will mail as a flat and cost accordingly.
Booklets and catalogs
When the message genuinely needs multiple pages - a product catalog, a program, a detailed guide - a saddle-stitched booklet is the format. It holds far more than any folded piece and reads like a small magazine, which is why retailers use product catalogs to drive repeat purchases and cross-sells.
Booklets and catalogs are almost always flats, and heavier ones can move into package prices, so postage is higher per piece. That is the deliberate trade: you spend more per piece to deliver much more content, so this format fits considered purchases, membership and donor communications, and catalogs rather than a single quick offer.
Dimensional mailers
A dimensional mailer - a piece with depth, a small box, or an unusual three dimensional shape - is used when standing out matters more than cost per piece. Dimensional mail draws attention precisely because it cannot be ignored in a stack of flat mail, and it is typically reserved for small, high-value audiences and specific segments where each response is worth a great deal, such as account-based outreach to key decision-makers. Because it leaves the letter and flat categories, dimensional mail carries the highest postage of any format, so the math only works when the audience is tightly targeted.
Every Door Direct Mail (EDDM)
Every Door Direct Mail is not a separate physical format so much as a way to send postcards or self-mailers to entire carrier routes without buying a mailing list. USPS offers EDDM as a low-cost route-based option, which makes it a favorite of local businesses covering the neighborhoods around a storefront, restaurant, or clinic. It trades the precision of a targeted list for saturation and simplicity. Whether route-based mail or a targeted list is the better fit is its own decision - this guide walks through it.
Door hangers: a related non-mail format
One specialty piece is worth naming because it is often grouped with direct mail: the door hanger. Door hangers are not mailed - they are hand-delivered to doors, so they skip USPS and its postage entirely. They are a field and canvassing tool, common in political mail and home services, rather than a mail format, but they solve the same problem of reaching a household with a printed message.
Pairing any format with digital
No format has to work alone. Adding a QR code to a postcard, letter, or brochure turns a physical piece into a measurable bridge to a landing page, and enrolling the mailing in USPS Informed Delivery shows subscribers a digital preview of the piece in their morning email the day it arrives. Personalized URLs and variable data printing let each recipient land on a page built around their name or offer. Used this way, direct mail becomes the anchor of a marketing campaign that runs across physical and digital channels at once, rather than a standalone drop.
How to choose the right direct mail format
Matching the format to the goal is most of the work. A workable way to narrow it down:
- Lowest cost, single offer, maximum exposure - a standard or oversized postcard, the cheapest addressed format, or EDDM to cover whole routes without a list.
- Personalization, a reply device, or a formal tone - a letter in an envelope, often as a full inserted package with variable data printing.
- More to say than a postcard, but envelope cost is not worth it - a folded self-mailer or brochure, kept within letter size and properly tabbed.
- Genuinely multi-page content - a booklet or catalog, accepting the higher flat postage.
- A small, high-value audience worth a splurge - a dimensional mailer.
The format sets the ceiling on your message and the floor on your cost, but the response still comes from the list, the offer, and the timing. Pick the least expensive format that can carry the message the offer requires, keep it within letter size unless the content truly needs a flat, and put the savings back into mailing more people. When you are ready to price a specific format against a real list, a postage estimate built on the actual sortation is the only way to see what each option really costs.