The swap that looks close
book wraps is the usual substitute for M-envelope boxes, and it is the wrong one. Using a book wrap because the M-envelope size is out changes the pack.
Packaging Boxes lists M-envelope boxes separately because the jobs diverge as soon as the goods are on the bench.
What the right line actually does
M-envelope boxes are the Tenvolope flat pack, described as 100% recycled and fully recyclable.
They are a different range from book wraps, which are E-flute with a peel-and-seal strip.
If you have already mixed them
A cube full of paper around a flat item is the long way round.
A Friday order on Packaging Boxes that includes M-envelope 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 M-envelope boxes when you already know this line. The page shows the pack quantity. Packaging Boxes will still dispatch an order placed by 3pm on the same day.