The swap that looks close
a brown cube is the usual substitute for white postal boxes, and it is the wrong one. A brown single wall carton can be the same size and the wrong finish.
Packaging Boxes lists White postal boxes separately because the jobs diverge as soon as the goods are on the bench.
What the right line actually does
White postal boxes are the white finish for a parcel that is meant to look like post, not like a warehouse carton.
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.
If you have already mixed them
A white box over the small parcel maximum is a dearer parcel with a white finish.
At Packaging Boxes, White postal boxes does not change the cutoff. Orders by 3pm leave the same day. Under £50 ex VAT the delivery is still available. It is just not the free threshold.
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.