The six crash lock sizes are a list. Goods that miss the list need a different box, not more fill. This page stays with white postal boxes, next to double wall boxes.
Packaging Boxes keeps White postal boxes apart from a brown cube. The category describes white postal boxes that qualify as a Royal Mail small parcel, in 125WK/T, some of them shell-and-slide with foam. The outside maximum still applies.
What this collects
White postal boxes are the white finish for a parcel that is meant to look like post, not like a warehouse carton.
double wall boxes is the neighbour on this list. Double wall is the stronger board, made in the UK, for heavy, fragile or stacked goods. It is still a different range from white postal boxes.
Before the order goes in
Stay inside the small parcel maximum the changes page gives as 450 x 350 x 160 mm on the outside. White does not raise that limit.
On Packaging Boxes, put White postal boxes on the order with the lines it belongs to, so you are not paying a second delivery for the piece you forgot. Free UK delivery starts over £50 ex VAT.