flat postage boxes

flat postage boxes are shallow cardboard boxes designed for slim items such as prints, documents, books, garments and flat packed products. Their low depth protects contents from bending and creasing far better than an envelope, while keeping the parcel within the slimmer Royal Mail size bands where possible. Most styles are supplied flat and fold together quickly at the packing bench, which saves storage space before use.

What flat postage boxes protect best

Anything that must arrive flat is a natural fit. Art prints, photographs, certificates, contracts, magazines, calendars and sewing patterns all suffer when an envelope folds or a sorting machine bends the pack. A shallow corrugated box holds the item rigid, protects the corners and gives a clean surface for the address label. Slim boxed goods such as phone cases, stationery sets and flat packed craft kits also travel well in this format.

Size names often follow paper formats, so A5 and A4 styles are common starting points. Our A5 postal boxes and C4 postal boxes cover two of the most requested document sizes, while letterbox postal boxes suit orders that should arrive without the customer waiting in.

Staying within Royal Mail bands with flat postage boxes

Royal Mail large letter post allows a maximum of 353mm in length, 250mm in width and 25mm in depth, with a weight limit that depends on the service used. A flat box with an internal depth below that limit keeps slim orders in the large letter band instead of the dearer small parcel band. Always measure the closed box, including any bulge from the contents, because the external size is what counts.

Board grades and closures for shallow boxes

Single wall corrugated board gives a good balance of rigidity and low weight for most flat items. White board presents printed documents and gifts neatly, while kraft board gives a natural look that many craft and book sellers prefer. Closures range from tuck in flaps that need no tape to adhesive strips for faster packing, and corners are the area to check for strength on larger sizes.

Packing flat items without damage

Place the item on a backing board if it is very flexible, keep print surfaces away from rough board with a paper sheet and avoid overfilling, because a lid forced shut will bow and can change the parcel depth. A fragile note on the outer box helps handlers keep the parcel flat in transit.

Custom sizes, print and ordering

Where a standard shallow size wastes space or leaves edges exposed, a custom size gives a closer fit and a more professional result. Print can add your logo, a do not bend message and returns information directly to the lid. For deeper orders that still begin life flat in storage, compare our large letter postal boxes before you decide.

Custom Postal Boxes manufactures in Wolverhampton with low minimum order quantities, free design support and free UK delivery. Pricing depends on size, material and quantity, so contact us for a quote with your item dimensions. Current Royal Mail bands are set out in the Royal Mail guidance on sizes and services, which is worth checking before a large print run fixes your box size.

FAQs about flat postage boxes

What can I send in a flat postage box?

Prints, documents, books, magazines, calendars, garments, stationery and slim boxed products all suit a flat postage box. The common feature is low depth, because the box protects the item from bending while keeping the parcel slim.

Will a flat postage box fit through a letterbox?

Many will, if the closed external size stays within the Royal Mail large letter limits of 353mm by 250mm by 25mm and the receiving letterbox is a standard size. Measure the closed box carefully and keep the contents flat so the lid does not bulge.

Are flat postage boxes supplied assembled?

No. They are normally supplied flat to save storage space and fold together quickly by hand when needed. Storing them flat and dry keeps the board crisp, which helps the finished box stay rigid in the post.

Can flat postage boxes be printed?

Yes. Lids can carry your logo, handling messages such as do not bend, and returns guidance. Custom Postal Boxes offers free design support, low minimum order quantities and free UK delivery on printed boxes made to your size.