The job it is listed for
A box list starts from the goods. White postal boxes are the white finish for a parcel that is meant to look like post, not like a warehouse carton. Packaging Boxes keeps White postal boxes as its own range so it is not picked as 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.
The line that is not a substitute
A brown single wall carton can be the same size and the wrong finish.
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.
Bringing it in
Count the white-box orders, not every parcel.
On Packaging Boxes, orders that include White postal boxes and are placed by 3pm are dispatched the same day. UK delivery is free on orders over £50 ex VAT. Stocked items can be returned within 14 days.
Open White postal boxes when you already know this line. Read the pack quantity. Packaging Boxes will still dispatch an order placed by 3pm on the same day.