Quick answer
Choose a letter pack (envelope, letter and inserts) when the message is personal, detailed or confidential, such as appeals, financial offers and renewals. Choose a postcard for short, simple messages where low cost and instant visibility matter. Choose a self-mailer (a folded, sealed mailer with no envelope) when you want more space and imagery than a postcard without the cost of enclosing. Size matters too: JICMAIL found that A4 mailers achieve 2.7 times the response rate of postcards, so weigh response as well as cost.
Key takeaways
- In warm retail mail, A4 mailers averaged a 9.6% response, A5 6.6% and postcards 3.5% (JICMAIL Response Rate Tracker, 2025).
- Letter packs offer privacy, room to persuade and multiple components (reply forms, vouchers, return envelopes).
- Postcards are the cheapest to produce and need no opening, but have little space and no privacy.
- Self-mailers fit in between: no envelope or enclosing, but more space and impact than a postcard.
- Postage depends on the Royal Mail size and weight band. Staying within the letter band (240 × 165 × 5mm, 100g) keeps costs down.
Letter vs postcard vs self-mailer at a glance
| Letter pack | Postcard | Self-mailer | |
|---|---|---|---|
| What it is | Envelope with a letter and optional inserts | Single card, message and address on the outside | Folded sheet sealed with glue or tabs, no envelope |
| Space for message | Most: multiple pages and inserts | Least | Medium to high, with multiple panels |
| Privacy | Yes, the content is enclosed | No, the message is visible | Mostly, as inner panels are hidden until opened |
| Personalisation | Full: letter, inserts and outer envelope | Name, address, offer and image on one side | Full, across panels |
| Response options | Reply forms, return envelopes, vouchers, QR codes | QR code, URL, phone or voucher code | Tear-off forms, QR codes, vouchers |
| Production | Printing plus folding and enclosing | Printing and trimming only | Printing, folding and sealing |
| Best for | Fundraising, financial services, renewals, B2B | Reminders, events, store openings, simple offers | Retail promotions, product launches, event programmes |
Does format affect response?
Yes. JICMAIL's Response Rate Tracker found that A4 mailers have 2.7 times higher response rates than postcards. In warm retail mail (634 campaigns in 2025), the averages were:
9.6%
A4 mailers: average response, warm retail direct mail
Source: JICMAIL, 2025
6.6%
A5 mailers: average response, warm retail direct mail
Source: JICMAIL, 2025
3.5%
Postcards: average response, warm retail direct mail
Source: JICMAIL, 2025
A bigger format gives more room to explain, show products and make the offer, and it is harder to overlook. It also costs more to print and may move into a higher postage band. The right question is which format gives the lowest cost per response for your audience, and testing is the best way to find out.
How format affects postage
| Royal Mail band | Maximum size | Maximum weight | Example formats |
|---|---|---|---|
| Letter | 240 × 165 × 5mm | 100g | DL or C5 envelope with folded A4 letter; A5 or DL postcard; folded self-mailer within the size limit |
| Large letter | 353 × 250 × 25mm | 750g | C4 envelope with flat A4 contents; catalogues; thicker self-mailers |
Large letters cost substantially more to post than letters. For example, Advertising Mail 2nd Class Mailmark Unsorted was 42.9p–44.8p for a letter and 85.5p–89.2p for a large letter (up to 100g) on the April 2026 rate card. A folded A4 letter in a C5 or DL envelope often gives you A4 impact at letter postage rates.
When to choose a letter pack
- The message needs explaining, as with a charity appeal, a financial product or a membership renewal.
- The content is personal or confidential.
- You want several components: a letter, a leaflet, a donation or order form and a reply envelope.
- You want the envelope to build curiosity with a teaser message or window.
Our enclosing lines use camera matching to make sure every personalised component goes into the right envelope. See UK direct mail.
When to choose a postcard
- The message is short: an event date, an appointment reminder, a store opening or a single offer.
- You want the lowest print cost and no enclosing.
- Speed matters. Postcards are quick to produce digitally, even when personalised.
- You are following up a previous mailing or email.
When to choose a self-mailer
- You need more space for products or images than a postcard allows.
- You want impact without the cost of envelopes and enclosing.
- You want a tear-off response form or voucher.
Tip
Self-mailers must be folded and sealed in a way that runs through Royal Mail's machines. Talk to us before artwork is finalised so we can check the fold, seal and address position.
Leaflets, mailers and inserts
Flat and folded print in-house, with folding, sealing and mailing under one roof.
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