The swap that looks close
an oversized cube is the usual substitute for Royal Mail small parcel boxes, and it is the wrong one. A larger single wall box may be stronger or roomier and still be a different, dearer format.
Packaging Boxes lists Royal Mail small parcel boxes separately because the jobs diverge as soon as the goods are on the bench.
What the right line actually does
These are the boxes aimed at the Royal Mail small parcel limit.
The outside maximum on the changes page is 450 x 350 x 160 mm. The box has to finish under that, tape included.
If you have already mixed them
A box that is fine until the tape and the bulge is not inside the limit.
TradePro at Packaging Boxes, for trade businesses, starts at 5% off and goes up to 10% when you spend over £1,000 a month, and it adds BACS. It does not change which Royal Mail small parcel boxes you needed.
Open Royal Mail small parcel boxes when you already know this line. The bundle count is on the page. Packaging Boxes will still dispatch an order placed by 3pm on the same day.