A bigger single wall box is not a stronger box. This page stays with Royal Mail large letter boxes.
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.
The notes below stay on Royal Mail large letter boxes. A small parcel box is the wrong economy if the goods would have gone as a large letter.
Before the order goes in
Check the format you actually post before you standardise.
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.