A custom mailer box, the most common format of custom shipping boxes, is a one-piece corrugated box with an integrated lid and self-locking closure that assembles without tape and protects the product in transit. It’s the standard e-commerce format because it combines strength, quick assembly and a surface you can print inside and out. Choosing well comes down to three decisions: dimensions, type of board and printing.
What exactly is a mailer box, and how is it different from other boxes?
The mailer box (also called a self-assembly box or tuck-top box) is die-cut from a sheet of corrugated board. You assemble it by folding the base, raising the walls and closing the lid, which has a tab that tucks into the front wall. It needs no tape or staples.
Compared with other shipping formats:
| Format | Assembly | Protection | Printing | Typical use |
|---|---|---|---|---|
| Mailer box | One piece, no tape | High, double side walls | Outside and inside, full surface | E-commerce orders, subscriptions, kits |
| Flap box (regular slotted carton) | Tape to close | High for heavy loads | Outside, usually one or two inks | Heavy shipments, logistics, B2B |
| Cardboard envelope | Adhesive closure | Low, flat products only | Outside | Books, folded textiles, documents |
| Box with separate lid | Two pieces | Medium | Outside and inside | Gifts; not suitable for shipping without an outer box |
The mailer box wins in e-commerce because it handles shipping and presentation at the same time. There’s no need for a nice box inside an ugly one: the shipping box itself is what the customer opens.
Dimensions: how to choose a size that saves on shipping
The costliest mistake with shipping boxes is ordering them too big. A box full of air costs more to ship, needs filling and makes the order feel small.
How to set the dimensions:
- Measure the product or set of products as they’ll travel, including any protectors.
- Add the minimum clearance: just enough to fit without forcing and without rattling. In general, between half a centimetre and one centimetre per side.
- Check your carrier’s rates. Many charge by volumetric weight bands. Taking off one or two centimetres can drop you into a lower band.
- Group orders into a few formats. It’s usual to cover the catalogue with two or three mailer box sizes. More formats make runs more expensive and complicate storage.
A detail that gets forgotten: mailer box dimensions are given as internal measurements. If you give external ones, the box will arrive slightly smaller than you expected. Confirm it with the supplier.
If you want to go further, we cover this in detail in the article on how to choose a shipping box size to pay less for transport. And our e-commerce shipping boxes page shows the most common formats.
Board: flute, paper weight and colour
Mailer boxes are almost always made from fine-flute corrugated board, which gives rigidity with little thickness and allows clean printing.
Type of flute
- Micro-flute (E or F flute). Very thin, good print surface, enough for light and medium products: cosmetics, textiles, accessories, books.
- B flute. Slightly thicker, more resistant to knocks and stacking. For heavier or more fragile products.
- Double-wall. For heavy loads or international shipments with lots of handling. Less common for mailer boxes because of the thickness.
Board colour
- Brown kraft. The cheapest and the most natural-looking. One-ink printing (black, white or a colour) works very well on it. Light colours and four-colour process lose intensity.
- White. A coated white outer surface allows faithful colours and photos. It costs a little more than kraft.
- White on both sides. To print the inside in vivid colours too. The most expensive option, and the one with the biggest impact on opening.
For most online stores, micro-flute kraft with one ink outside and one ink inside gives a polished result at a contained cost. White is justified when the brand relies on precise colours or images.
Printing: outside, inside and what really gets seen
A mailer box has two brand faces: the outside, seen by the courier and the neighbour, and the inside, seen by the customer on opening. They’re two different moments and don’t need the same treatment.
- Outside. It should identify the brand and withstand rough handling in transit. A logo, a colour and perhaps a phrase. Saturated backgrounds scuff and mark with friction.
- Inside. This is where the emotion is. A flat colour, a pattern, a welcome message or instructions. It’s what gets photographed and what turns a delivery into a brand experience.
- Inks. One ink outside and one inside is the most efficient combination. Four-colour process makes sense for images or gradients on white board.
A budget-friendly approach: print only the inside in colour and leave the outside kraft with a small logo. The parcel goes unnoticed in the van (less unwanted attention) and surprises on opening. We go into more detail in our guide to e-commerce packaging.
Inserts and extras: protecting without filling
A well-sized mailer box needs little filling. When the product is fragile or there are several products, the options are:
- Die-cut cardboard insert. A piece that holds each product in its own slot. The cleanest solution and the best presentation.
- Tissue paper or crumpled kraft. Cheap and recyclable. Gives a gift feel at little cost.
- Card or flyer. Thank-you note, instructions, a discount code for the next purchase.
- Closure label. A sticker on the lid that seals the box and adds colour. With a sticker label you can change the message for each campaign without reprinting boxes.
Avoid plastic fill if your brand talks about sustainability; customers notice and comment.
What makes a custom mailer box more expensive, and how to compare quotes
The factors that move the price most, in general:
- Size. More board per unit. This is the first factor.
- Type of board. White versus kraft, B flute versus micro-flute, two coated sides versus one.
- Printing. Number of inks, printed sides and coverage. Printing the inside adds a step.
- Cutting die. If the format is custom, there’s an initial die cost that’s paid off over the run.
- Print run. The unit cost drops a lot as units increase, because fixed costs are spread out.
- Inserts. Every inner piece is an extra die and an extra process.
To compare quotes between suppliers, fix the internal dimensions, the type of board and flute, the colour, the inks per side and the print run. If one quote is much lower on the same specifications, that’s margin. If the specifications aren’t the same, it isn’t comparable.
If you already have a quote for mailer boxes or shipping boxes for your online store, upload it here and within 48 business hours we’ll tell you how much you can save with the same board, the same inks and the same print run. If we can’t improve it, we tell you and you pay nothing.
Frequently asked questions
What’s the difference between a mailer box and a regular cardboard box?
A mailer box is one piece, with an integrated lid and self-locking closure, and assembles without tape. It’s designed so the shipping box itself is the presentation box. A traditional flap box needs tape and protects heavy loads well, but isn’t designed for the unboxing.
Kraft or white for custom shipping boxes?
Kraft is cheaper and works very well with one ink. White allows faithful colours and photos, and makes sense if your brand depends on an exact colour or on images. For most stores, kraft with one ink outside and another inside is enough.
How many mailer box sizes does an online store need?
Usually two or three cover almost the whole catalogue. More formats make runs more expensive and complicate storage; fewer force you to use large boxes for small orders, which pushes up shipping and filling costs.