The lines that belong together
Order white boxes with clear tape. Brown tape marks them.
Foam in a shell-and-slide is part of that product. It is not a reason to skip a size check.
The line that stays separate
a brown cube stays on its own order line. A brown single wall carton can be the same size and the wrong finish.
White postal boxes is the range Packaging Boxes uses for white postal boxes.
One basket
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.
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.