Measure before you open the range
Check the format you actually post before you standardise.
Large letter boxes are for the large-letter format, not for a small parcel that happens to be flat. The page for Large letter boxes is where Packaging Boxes shows that line.
The published limit, if there is one
They are a separate range from small parcel boxes and from book wraps.
A small parcel box is the wrong economy if the goods would have gone as a large letter.
Write the size down
Say large letter and the size on the reorder.
At Packaging Boxes, Large letter boxes does not change the cutoff. Orders by 3pm leave the same day. Under £50 ex VAT the delivery is still available. It is just not the free threshold.
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.