Large letter and small parcel are formats. Choosing the larger one “to be safe” pays the larger one every time. This page stays with Royal Mail large letter boxes, next to book wraps.
Packaging Boxes keeps Large letter boxes apart from a small parcel box. They are a separate range from small parcel boxes and from book wraps.
What this collects
Large letter boxes are for the large-letter format, not for a small parcel that happens to be flat.
book wraps is the neighbour on this list. Book wraps are the flat pack for a book or a slim parcel. It is still a different range from Royal Mail large letter boxes.
Before the order goes in
Check the format you actually post before you standardise.
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 Large letter boxes you needed.