The swap that looks close
a square box is the usual substitute for postal tubes, and it is the wrong one. A square box needs fill in the corners and still presses a roll.
Packaging Boxes lists Postal tubes separately because the jobs diverge as soon as the goods are on the bench.
What the right line actually does
A postal tube takes a roll without crushing it into a square.
Tubes are their own range, for posters, plans and other rolls.
If you have already mixed them
Folding the roll into a box creases it.
On Packaging Boxes, put Postal tubes on the order with the lines it belongs to, so you are not paying a second delivery for the piece you forgot. Free UK delivery starts over £50 ex VAT.
Open Postal tubes when you already know this line. Check how many tubes are in the pack. Packaging Boxes will still dispatch an order placed by 3pm on the same day.