C4 PiP boxes

C4 PiP boxes are slim postal boxes sized for A4 contents and built around Royal Mail Pricing in Proportion rules, so a packed parcel can qualify as a large letter rather than a small parcel. A large letter may measure up to 353 mm by 250 mm by 25 mm and weigh up to 750 g. At Custom Postal Boxes, a project of Vertex Packaging in Wolverhampton, C4 PiP boxes are made in house with custom print options, low minimum order quantities, free design support and free UK delivery.

How C4 PiP boxes keep A4 post in the large letter band

Pricing in Proportion means Royal Mail charges by the size of the item as well as its weight, so a few millimetres of extra depth can move a parcel into a more expensive band. C4 PiP boxes are designed with a shallow depth, commonly around 20 mm internally, which leaves room for the board and the seal while keeping the finished external depth inside the 25 mm large letter limit. The C4 name comes from the envelope size that holds a flat A4 sheet, and the box follows the same logic with rigid protection.

Discipline matters when packing. An overfilled box that bulges at the lid can measure over the limit even if the empty box was correct, and a thick wad of tape or a raised label adds depth too. Pack contents flat, spread any thickness evenly and measure the sealed parcel rather than relying on the empty box dimensions.

Styles of C4 PiP boxes and what fits inside

The format is usually made as a shallow fold up postal box or as a wrap style box that folds around the contents. Both give flat A4 items far more protection than an envelope, because the corrugated board resists bending and shields the corners. Typical contents include certificates, calendars, art prints, magazines, slim paperback books, documents in presentation folders and flat packed garments or accessories.

Protecting flat contents from bent corners

Corners are the weak point of flat post. A rigid box holds the corners square, and adding a stiff backing board inside spreads any pressure across the whole item instead of one edge. Keep contents centred with a small amount of room at each edge, and avoid forcing in an item that is fractionally too wide, because a strained fold can spring open in transit.

Sealing without adding bulk

Seal along the designed closure and keep tape flat and close to the surface. One neat strip placed centrally usually holds a shallow box securely, while tape wrapped repeatedly around the depth adds millimetres exactly where the size limit is tightest. Place the address label flat on the largest panel so it cannot lift a corner of the lid.

Checking size and weight before you order

Weigh the fully packed parcel, including the box, contents and any inserts, and keep the total under 750 g for large letter services. The Royal Mail guidance on sending parcels explains how size and weight decide the service you can use, and it is worth checking current limits there before a large mailing. For related sizes, compare C4 postal boxes, large letter postal boxes and letterbox postal boxes, with A5 postal boxes covering the smaller C5 format for A5 contents.

Custom print and ordering

A shallow box still offers a generous print area on its lid, so logos, handling notes and return details can all be carried clearly. Ordering is quote based and pricing depends on size, material and quantity. Send your A4 contents, target quantity and print ideas through the contact page for a quote and the team will confirm the box specification with free design support before anything is made.

FAQs about C4 PiP boxes

What is the maximum size for C4 PiP boxes sent as a large letter?

The packed parcel must stay within the Royal Mail large letter limits of 353 mm in length, 250 mm in width and 25 mm in depth, and it must weigh no more than 750 g. Measure the sealed parcel rather than the empty box, because tape, labels and bulging contents all count towards the final size.

What can I send in C4 PiP boxes?

They suit flat A4 sized contents such as certificates, calendars, art prints, magazines, documents, slim paperback books and flat packed accessories. Anything deeper than the shallow internal space, or heavy enough to push the packed weight over 750 g, belongs in a small parcel box instead.

What is the difference between C4 and C5 PiP boxes?

C4 is the larger format and holds an A4 sheet flat without folding, while C5 holds an A5 sheet or an A4 sheet folded once in half. Choose C4 when the contents must stay completely flat at full A4 size, and C5 for smaller documents and booklets.

Can C4 PiP boxes be custom printed?

Yes. The lid and sides can carry your logo, artwork, handling guidance and return information, and printing inside the lid is also possible. Free design support is included, so artwork is checked against the box template before production.

How do I order C4 PiP boxes?

Contact the team with the size of your contents, how many boxes you need and any print requirements. Ordering is quote based, with pricing set by size, material and quantity, and delivery within the UK is free.