Custom Side-Loading Mailers

Board Grades and Protection

Most side-loading mailers are made from E-flute corrugated board, which is thin enough to keep the box slim but strong enough to stop corners being crushed. For heavier books or framed prints, B-flute gives more stiffness and edge protection. Double-wall board is available for very heavy or high-value items.

Kraft corrugated is the most economical choice and hides marks from postal handling. White corrugated gives brighter print and a cleaner look when the parcel arrives, which suits publishers and art sellers who want a polished unboxing. Corners are the weak point in any flat mailer, so we size the box to keep the item snug and avoid empty space at the ends.

If the item is valuable or easily bent, such as a signed print, consider a stiffening board inserted alongside it. The board spreads any pressure across the whole face and stops creasing if the parcel is bent in a postbag. You can see B-flute and double wall explained in our board guide.

Sizing for a Snug Fit

Measure the length, width and thickness of the item, including any shrink wrap or protective sleeve. Add 3 to 5 mm on the length and width and a few millimetres on the depth. A snug fit stops the item sliding and keeps the box rigid. Too much space lets the item move and puts stress on the corners.

If you send several items of slightly different sizes, such as books of varying thickness, a box with a crease line that allows a shallower or deeper fold can cover a range. We can also supply card inserts to pad out thinner items in a larger box.

Check the final size against postal size bands before ordering. A box that is a few millimetres slimmer can sometimes fit a lower price band, which adds up quickly over hundreds of parcels.

Thickness is the dimension most often underestimated. A hardback measured without its dust jacket, or a print measured without its backing board, can make the box too tight to load. Measure the item exactly as it will be packed. Our guide on how to measure a framed print covers glass and hanging fittings.

Printing and Branding

Side-loading mailers are often printed on the top face with a logo and return address, or all over for a fully branded look. Publishers frequently print the book cover artwork or a series pattern on the box. Art sellers add fragile handling notes and a do not bend message, which helps postal staff handle the parcel with care.

Inside print is possible too. A short message, care instructions or a discount code inside the end flap gives customers something to read as they slide the item out.

Branding does not need to be elaborate. A single colour logo on kraft board, repeated on both faces, is inexpensive and clearly identifies the parcel. Full colour all-over print suits special editions and gift releases, where the box is part of the product.

Record labels often match the mailer to the album artwork for limited pressings, and the box becomes part of the collection. For everyday orders, a plain kraft box with a simple logo does the job at a lower cost.

Packing Tips From Busy Despatch Benches

Keep mailers flat and stacked near the packing bench, folded only as needed. Pre-folding a small batch at the start of the day saves time when orders start coming in.

Slide the item in with the spine or the strongest edge leading, so the softer edges sit away from the closing flap. If you include a delivery note or invoice, place it on top of the item before sliding it in, so it comes out first.

For high-value orders, weigh a packed sample and photograph it before sealing. It is a simple habit that helps resolve any queries about what was sent. Galleries posting unframed work use art print shipping boxes with a backing board in the same style.

Measure the Item Flat and Add Only a Few Millimetres

Side-loading mailers work because the item slides in snugly, so the measurement has to be tight. Lay the book, print or frame flat and measure length, width and thickness, then add 3 to 5 mm to the length and width and only the thickness of any sleeve or backing board. If several thicknesses share one footprint, ask for adjustable crease lines rather than a box per title. Say whether the mailer must fit a large letter band, since that caps the depth, and whether the item is glass-fronted, because a framed print needs corner protectors inside the mailer and B-flute board. A do-not-bend message on both faces is a cheap and useful addition.

Send the item dimensions, quantity and print, and a custom quote comes by email within one working day. The free proof shows the mailer flat and folded with the tab lock marked. Production takes 4 to 7 working days after approval, mailers arrive flat by tracked courier to UK mainland addresses, and misprints or damage reported within 10 days are reprinted or refunded.