A sheet, not a carton
Cardboard layer pads are single wall corrugated sheets. The category also calls them pallet pads, and describes them as about 2.5 mm thick, B flute 125K/T. They separate layers on a pallet. They are also used as backing for arts, crafts, posters and wrapping.
A removal carton and a postal box are different products. The Royal Mail small parcel maximum of 450 x 350 x 160 mm is the wrong test for a flat sheet.
What sits beside the pad
Corrugated paper is a single face roll, lightweight, for wrapping goods in storage or transit, and it is 100% recycled, recyclable and biodegradable. Jiffy foam is a soft CFC-free layer for glassware, crockery, cutlery and electronics. Neither roll is the 2.5 mm pad.
Orders placed by 3pm are dispatched the same day. UK delivery is free on orders over £50 ex VAT.