The swap that looks close
a small parcel box is the usual substitute for Royal Mail large letter boxes, and it is the wrong one. A small parcel box is the wrong economy if the goods would have gone as a large letter.
Packaging Boxes lists Large letter boxes separately because the jobs diverge as soon as the goods are on the bench.
What the right line actually does
Large letter boxes are for the large-letter format, not for a small parcel that happens to be flat.
They are a separate range from small parcel boxes and from book wraps.
If you have already mixed them
Choosing the small parcel box “to be safe” pays the larger format every time.
On Packaging Boxes, orders that include Large letter boxes and are placed by 3pm are dispatched the same day. UK delivery is free on orders over £50 ex VAT. Stocked items can be returned within 14 days.
Open Large letter boxes when you already know this line. Read the pack count. Packaging Boxes will still dispatch an order placed by 3pm on the same day.