Postal Boxes & Mailers

Source: Royal Mail size and weight guide. Our sizes are internal dimensions, so check the external size of any box close to a limit before choosing the band. Long Postal 635mm is over the Medium Parcel limit, so send it by courier. The weight limit includes the contents.

Postage is usually the second biggest cost in a UK e-commerce operation, after the product itself. The mailer you choose decides which Royal Mail band you pay for — and getting that wrong by a few millimeters costs you on every single order.

This range is built around the bands, not around box shapes. Everything here is sized to post: Large Letter formats that go through a letterbox, and Small Parcel formats for anything thicker. All are sold by the pack for UK trade and online sellers, with a £75 minimum order.

If you're looking for boxes to pack and protect rather than to post — single wall and double wall corrugated — those sit in our cardboard boxes range. Flat envelope-style mailers are in envelopes.

Start with the Royal Mail band, not the box

Royal Mail prices on dimensions first and weight second. There are three bands that matter for most sellers:

Every dimension has to pass, not just the ones you care about. A mailer 30cm x 22cm x 3cm fails Large Letter on depth alone and is charged as a Small Parcel — despite being flat enough to post through most letterboxes.

Work backwards: measure your product, decide which band it can realistically hit, then pick the mailer that keeps you inside it.

Which mailer for which item

The 2.5cm rule: where most sellers lose money

The Large Letter depth limit is 2.5cm. It is the single most expensive line in the Royal Mail price list to cross, because the jump from Large Letter to Small Parcel is a step change, not a small increment.

Three things catch sellers out:

Measuring empty, not packed. A mailer that measures 2.3cm flat will bulge past 2.5cm once there's a product, tissue and a returns slip inside it. Measure it packed, as it will actually go out.

Ignoring the 750g weight cap. A mailer can pass on all three dimensions and still be charged as a parcel because the contents tip over 750g. Both tests have to pass.

Assuming "letterbox-friendly" means Large Letter compliant. They aren't the same claim. A box can fit through a letterbox and still be too deep or too heavy for the band. Check the dimensions, not the description.

If your product genuinely won't fit inside 2.5cm, stop optimising for Large Letter and buy properly for Small Parcel instead. A well-sized parcel mailer beats an over-stuffed letter format that arrives damaged.

Variable-depth formats, and why they're worth it

Book wraps and wrap-around mailers fold to the depth of the contents rather than a fixed height. For a bookseller or record shop with stock in dozens of thicknesses, that means one pack covers a wide range instead of holding four fixed sizes in stock.

Two practical benefits: you carry less packaging inventory, and you stop paying parcel rates on items that could have been Large Letter if the box had been the right depth. The trade-off is a slightly slower pack time than a rigid box that snaps together.

Medium-duty handles most books, media and flat goods. Heavy-duty is worth the extra for hardbacks, higher-value items, or anything you've had damage claims on.

Peel-and-seal, and the cost you're not counting

Most mailers in this range use a peel-and-seal strip rather than tape. It's easy to dismiss as a nice-to-have, but it's a labour cost: at a few seconds saved per parcel, a business dispatching 200 orders a week gets hours back over a month, plus no tape gun and no tape consumable.

If you're packing at volume, factor pack time into your cost per order alongside the unit price. The cheapest mailer per unit isn't always the cheapest to send.

Buying by the pack: trade pricing

Everything here is supplied in trade pack quantities — 40, 50, 60 or 100 depending on format — with a £75 minimum order. Prices shown exclude VAT, which is added at checkout.

A common mistake at this stage is buying one pack of six different sizes. You end up with capital tied up in slow-moving formats and a picking bench nobody can find anything on. Two or three formats that cover 80% of your dispatch will almost always beat six that cover 100% — and the volume you concentrate into fewer lines is what earns the better break price.

If you're not sure which formats those are for your product mix, get in touch with your dimensions and typical weights and we'll work it through with you.

Still deciding?

• Need protection more than postage efficiency? See cardboard boxes for single and double wall.
• Sending documents or flat card? See envelopes.
• Sending mugs, ceramics or bouquets? Those specialist formats are in cardboard boxes.