General description Royal Mail Large Letter PIP Cardboard Postal Boxes are high-quality, UK-manufactured corrugated boxes designed specifically for the Royal Mail 'large letter' postal system. These brown and white single wall die cut boxes are easy to assemble, featuring fold-over ends and "earlock" lids, making them ideal for secure packaging. Technical specifications C5 Size - Internal: 222x160x20mm C5 Size - External: 228x164x22mm C6 Size - Internal: 160x110x20mm C6 Size - External: 166x114x22mm C4 Size - Internal: 332x245x22mm C4 Size - External: 332x245x24mm