Letterbox Boxes

Letterbox boxes and letterbox postal boxes for UK subscription brands, ecommerce sellers and direct-to-consumer retailers. 20mm deep, rated for Royal Mail Large Letter, and sized to post through the door rather than sit in a depot. Also called Large Letter boxes, PiP boxes or letterbox size boxes. Made on our own corrugated plant, supplied flat in trade packs.

Two different claims, and only one of them is about postage

Almost every supplier conflates these. They're separate, and getting them mixed up costs money in different ways.

Royal Mail Large Letter is a pricing band: 353 × 250 × 25mm, up to 750g. Stay inside it and you pay letter rates instead of parcel rates. Every box on this page is inside it — the 20mm depth leaves 5mm of tolerance on the most-constrained dimension.

Fitting through a letterbox is a physical question, and the limit is the aperture, not the band. BS EN 13724 sets a minimum recommended slot of 230 × 30mm, and plenty of UK doors are at or near that minimum.

So the two largest sizes behave differently. Everything up to 225mm wide goes through a minimum-spec slot. Our widest box, at 305mm, does not — it's correctly rated for Large Letter postage, but it needs a wider-than-minimum aperture to post through the door.

If letterbox delivery is the point of your product, size at 225mm or below. If you just want Large Letter postage rates and don't mind a doorstep handover, the widest size is the most capacity you can get inside the band.

Why 20mm, and why depth is the dimension that matters

Large Letter allows 25mm. These are 20mm, and the 5mm gap is deliberate.

A box measured flat and a box packed are not the same object. Contents bulge, seals add thickness, and a box built to exactly 25mm will be over it in practice — at which point Royal Mail reprices the whole consignment as a parcel and the saving disappears.

Twenty millimetres gives you working tolerance. It also clears the 30mm minimum aperture with room to spare, which is what makes reliable letterbox delivery possible rather than occasional.

The trade-off is real and worth stating. Twenty millimetres is not much depth. Soft goods that compress — clothing, coffee, cosmetics in flat packaging, printed matter — work well. Anything rigid and over about 15mm does not, and no amount of choosing a bigger footprint solves it.

What letterbox delivery is actually worth

The postage saving is the obvious part. The operational saving is usually bigger.

No missed delivery. No card, no redelivery, no depot collection, no customer service email asking where the parcel is. For a subscription business, a missed delivery on a monthly box is a churn risk, not just a cost.

No delivery window. The customer doesn't wait in. For gifting and for anything time-sensitive that's a real difference in experience.

Fewer "not received" claims. A parcel left in a porch goes missing. A parcel through the door usually doesn't.

Those three together are why subscription brands design their whole product around the 20mm limit rather than treating packaging as an afterthought.

Choosing a size

Footprints and pack quantities are in the table below. Three things decide it.

Do you need through-the-door, or just the postage band? Answered above, and it's the first question because it rules out the widest size for one of the two uses.

Measure the product packed, not flat. A 20mm box takes a product up to about 15–17mm once you allow for the board and the contents settling. Make one up and try it before you order a pallet.

Take the smallest footprint that works. A product moving around inside an oversized letterbox box arrives looking worse than the same product in a snug one, and there's no room for void fill in 20mm.

What these suit

• Subscription boxes — coffee, tea, snacks, cosmetics, stationery, pet treats
• Cosmetics and skincare in flat or sachet packaging
• Jewellery and small accessories
• Prints, cards and stationery
• Supplements and nutraceuticals in blister or sachet form
• Socks, underwear and flat apparel
• Small electronics and cables
• Samples, testers and promotional inserts

Why buy from us

We're part of a manufacturing group and the corrugated board in these boxes is made in-house rather than bought in and marked up. On a product where the whole business case is postage cost, the packaging cost matters too — and there's no middleman margin in what you pay.

Making our own board also means the specification is negotiable. If your product needs a footprint we don't stock, or a printed outer, that's a bespoke conversation rather than a no.

Practical detail

Single-wall corrugated, supplied flat and pre-scored for fast assembly, with peel-and-seal closure for tape-free packing. The kraft surface takes direct print and labels cleanly — worth considering on a subscription product, where the box is the first thing the customer sees every month. Fully recyclable in normal paper collection.

Not sure whether your product clears 20mm? Send us the dimensions and we'll tell you straight.

Delivery and returns

Next-day dispatch on UK mainland orders, with free UK mainland delivery and a £75 minimum order. Full terms on our delivery and returns pages.