Postpak Red Full-Shirt Small Mail Box, 450mm x 350mm x 80mm - Pack of 20. The PostPak Full-Shirt Small Mail Box is a heavy duty, strong and reliable cardboard box. PostPak's convenient pop-up design includes self locking tabs located from the lid and a peel and seal for additional parcel item security. This mailing box is designed to conform to Royal Mail packaging standards and minimise postage costs.