The job it is listed for
A box list starts from the goods. Large letter boxes are for the large-letter format, not for a small parcel that happens to be flat. Packaging Boxes keeps Large letter boxes as its own range so it is not picked as a small parcel box.
They are a separate range from small parcel boxes and from book wraps.
The line that is not a substitute
A small parcel box is the wrong economy if the goods would have gone as a large letter.
Check the format you actually post before you standardise.
Bringing it in
The quantity is the large-letter orders only.
A Friday order on Packaging Boxes that includes Large letter 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 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.