Adding Inserts and Flyers to Letterbox Packaging Without Increasing Size
Include marketing materials in letterbox packaging without exceeding size limits or adding postage cost.
The size challenge
Letterbox boxes must stay under 25mm thick to qualify for Royal Mail large letter pricing. Every millimetre counts. Adding inserts, flyers, or thank-you cards risks pushing the total thickness over the limit, which bumps your postage up to the much more expensive small parcel rate.
Quick answer
Use thin card stock (under 350gsm) for inserts, print on lightweight paper for flyers, and design flat items that add less than 1-2mm to total thickness. Place inserts on top of the product, not beside it, to avoid adding width. Measure the packed box with inserts included before committing to a bulk print run. A postcard-sized insert on 300gsm card adds roughly 0.4mm.
Insert options that fit
Small, thin, and impactful. A branded postcard with a discount code adds under 0.5mm.
A single-sheet flyer adds negligible thickness and can carry substantial marketing content.
A branded sticker sheet adds virtually no thickness and customers love them.
You absolutely can - the key is choosing thin materials and measuring the total packed thickness.
Sizing rules: Royal Mail letterbox rules or choosing the right size.
Keeping within limits
Insert guidelines0 / 4 checked
Maximising marketing impact
The most effective letterbox inserts are a discount code for the next purchase (drives repeat orders), a QR code linking to your website or review page, and your social media handles. Keep the design simple and the call to action clear. One insert with a strong message outperforms multiple items that feel like junk mail.
Letterbox boxes with room for inserts
ProcuraPack letterbox boxes designed to maximise product space while leaving room for marketing materials.
Browse letterbox boxesFrequently asked questions
Yes, as long as the total packed thickness stays under 25mm for Royal Mail large letter pricing.
A postcard on 300gsm card adds about 0.4mm. A single-sheet flyer on 120gsm adds less than 0.2mm.
A branded postcard with a discount code for the next order. High impact, minimal thickness.
Only if you add too many or use thick materials. Always measure the total packed box before committing.
Both work. Stickers add almost zero thickness and are popular with customers. Postcards carry more information.
Part of our guide
Product Guide: Letterbox Boxes
Expert guides on letterbox packaging, from sizing and Royal Mail rules to branding and sustainability.
Browse all guides →
