Measure before you open the range
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.
White postal boxes are the white finish for a parcel that is meant to look like post, not like a warehouse carton. The page for White postal boxes is where Packaging Boxes shows that line.
The published limit, if there is one
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.
A brown single wall carton can be the same size and the wrong finish.
Write the size down
Reorder the white size, and say white.
A Friday order on Packaging Boxes that includes White postal boxes aims for Monday. A bank holiday on Friday or Monday moves that delivery. The 3pm cutoff is still the same-day dispatch rule.
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.