Three boxes that are not a cube
An archive box is an A4 carton with handles and an integral lid, supplied flat. The sides are triple thickness once it is made up, for files that will be stacked. A wardrobe box includes a plastic rail so clothes hang instead of folding. A white postal box is a 125WK/T outer, including shell-and-slide packs with foam, and the category says those white postal boxes qualify as Royal Mail small parcels.
A standard single wall cube does none of those three jobs as well. The file box is for the shelf. The wardrobe box is for the hanging rail. The white box is for how the parcel looks.
The postal limit still applies
White does not change the Royal Mail small parcel maximum. Our changes page gives that as 450 x 350 x 160 mm on the outside. A white box over that size is a dearer parcel.
Orders placed by 3pm are dispatched the same day. UK delivery is free on orders over £50 ex VAT.