custom letterbox boxes

custom letterbox boxes are slim postal boxes made to a chosen size and printed design so flat products can be delivered through a standard letterbox without the customer waiting at home. Custom Postal Boxes, a project of Vertex Packaging in Wolverhampton, makes custom letterbox boxes in house for UK businesses with low minimum order quantities, free design support and free UK delivery.

Why custom letterbox boxes improve delivery success

A parcel that fits through the letterbox avoids a missed delivery, a doorstep wait and often a second journey to collect it. That matters for repeat orders such as refills, subscriptions and small gifts, where convenience is part of the product. A custom size goes further than a stock slim box because the footprint can match the product closely, reducing movement inside and giving a tidier opening experience.

The postal limit still controls the design. Royal Mail large letter guidance sets a maximum of 353 mm long, 250 mm wide and 25 mm thick with a weight limit of 750 g, and you can check the current figures on the Royal Mail small parcel and letter size guidance. Remember that the limit applies to the finished packed parcel, including board thickness, tape and any raised closure, not only to the product inside.

Products that suit a slim letterbox format

Flat and fairly light products work best, including prints, documents, books, greeting card sets, small clothing items, beauty sachets, phone accessories and sample packs. Products with a rigid component need a box that resists bending, while soft items may still need a board stiffener so edges do not fold in transit. If an item is close to the thickness limit, pack a trial parcel and measure it again once sealed.

Leaving a safety margin below the maximum size

Designing exactly to the maximum leaves no room for board thickness, a label fold or slight bulging when the box is filled. A small margin below the limit is safer in daily packing, where speed can add tape or tissue that changes the final depth. Record the packed dimensions on your packing notes so every packer works to the same checked size.

Design, board and print choices

Start with the formats in our letterbox postal boxes range, then compare large letter postal boxes for Royal Mail large letter work and A5 postal boxes where an A5 footprint is the natural fit. For larger flat documents, C4 postal boxes show the next size step.

Board choice is a balance between slimness and strength. A thin corrugated board gives cushioning and keeps its shape, while a solid board may suit very light flat items. White board gives a bright print base for colour artwork, and kraft board gives a natural look that suits bold simple designs. Print on the lid is the first brand contact, and a short message inside can reward the customer for opening the box.

Packing slim products without damage

Keep the product flat and central, add a thin protective layer over printed or delicate surfaces and avoid loose fill that pushes the lid upward. For items with corners, such as prints and books, corner protection or a snug fit matters more than thick padding. Test the packed box by sliding it through a letterbox sized opening if you can, rather than relying only on a tape measure.

Ordering custom letterbox boxes

Pricing depends on size, material and quantity, so this range is quote based with no stated price. Tell us the product dimensions, the packed weight, whether you need the parcel to stay within large letter limits and the quantity you expect to use. Free design support can check that logos and text stay clear of folds and that the artwork fits the slim lid area well.

To begin, send those details through our contact page for a quote and mention any delivery promise you make to customers. Low minimum order quantities let you trial a custom size on real orders, check customer feedback and adjust the fit before you commit to a larger quantity for a busy season.

custom letterbox boxes FAQs

What size should custom letterbox boxes be for Royal Mail large letter post?

The finished packed parcel must be no more than 353 mm long, 250 mm wide and 25 mm thick and weigh no more than 750 g to stay within Royal Mail large letter limits. Measure the sealed box rather than the product alone, and leave a small margin for tape and board thickness.

Will a custom letterbox box definitely fit every letterbox?

Letterboxes vary in size, especially on older doors, so no box can be guaranteed to fit every single letterbox. Designing within Royal Mail large letter limits and keeping the box slim gives the best chance of delivery through a standard letterbox.

Can custom letterbox boxes be printed inside and outside?

Yes, print can be placed on the outside, inside the lid or both, depending on the construction. Many brands keep the outside simple for posting and use the inside lid for a message, instructions or a repeat order prompt.

What if my product is slightly too thick for a letterbox box?

If the packed depth exceeds 25 mm or the weight exceeds 750 g, the item moves into a parcel format instead of large letter. A custom slim box may still help delivery, but you should plan postage and delivery promises around the correct parcel category.