Custom Side Loading Mailers Made to Your Size

What is a side loading mailer?

It is a flat-packed corrugated mailer that is built around the item and closed at one short end. Instead of a lid on top, the opening sits at the side, where the item slides in.

Our photos show the classic white version. One shows a banded bundle of flats ready to use. Two show a folded box with the end open and a small tuck flap standing up. Another shows a pale blue printed pair, one sliding out of the other.

The style is close to our slim literature mailers for documents, but usually a little deeper and sturdier.

Why load from the side?

Flat items are awkward to lower into a top opening box. They catch on the walls, and the lid often sits loose above them.

With a side loader, the item slides along the base and the box closes tight around it. The top panel is a solid sheet with no seams, so it gives a clean surface for an address label or a printed logo. Packers on a long run often find it faster, because the box can sit flat on the bench the whole time.

There is also less board in the build. With no double walls or deep lid, a side loader uses material only where the item needs it. That keeps the parcel light and slim, which can help it fit through letterboxes and parcel lockers.

How do I size a side loading mailer?

Measure the item's length, width and thickness. Add a few millimetres to the length and width so it slides in easily, and a little height if you wrap it first.

Getting the height right matters most here. Too much space lets the item rattle; too little makes it hard to slide in. For framed items, include the corner protectors in your sizes. Our page on common UK box sizes lists typical formats if you are working from paper sizes.

If you post more than one thickness, such as single books and pairs, tell us both. It is often cheaper to have two heights than to fill a deep box with paper.

What board do side loaders use?

Most are single wall corrugated, white or kraft. The choice depends on weight and how much protection the edges need.

For most single books and prints, E flute is plenty. Step up to B flute when the item has hard corners, such as a frame, or when several books are packed together.

Can side loading mailers be printed?

Yes. The large flat top panel is ideal for a logo or a simple pattern. We print plain, one colour, two colour or full colour CMYK, and can print on the inside too.

White board shows colour cleanly, as in the pale blue example in our photos. Kraft gives a warmer look that works with one colour black or white ink. Lamination, spot UV, foil and embossing are available, but many sellers keep side loaders plain or one colour, as they are mostly a shipping box. Our designers give free layout help and send a free digital proof.

Side loader or padded envelope: which is safer?

A padded envelope is light and cheap, but it bends. Anything that must arrive flat, such as a print, a certificate or a hardback, can crease or dent inside one.

A side loading mailer gives a rigid shell on all sides. Corrugated board resists bending far better than paper padding, and the closed ends protect the corners. It weighs a little more, so postage may change, but returns from bent goods cost far more than the extra grams.

Some sellers use both: envelopes for soft items like scarves, and side loaders for anything flat and stiff.

Mistakes to avoid

The most common mistake is making the box too deep. A side loader works because it holds the item tightly, and extra height undoes that.

Another is skipping tape on the end. The tuck flap holds well on a desk, but sorting machines and bags can push it open. Finally, avoid placing the address label across the opening, as the customer will have to cut through it.

It also pays to check the proof for the open end. Make sure no text or logo crosses the tuck flap fold, where it will be hidden or split once the box is closed.

How to order side loading mailers

Side loaders are priced from £0.25 per unit. The final figure depends on size, board and print, and we put it in writing for you.

Send your item sizes through our side loader quote request, or email info@ordercardboardboxes.co.uk. We proof first, then make your mailers in 4 to 6 business days. Tracked delivery takes 2 to 3 business days and is free to any UK address.