The swap that looks close
a folded mailer is the usual substitute for board-back envelopes, and it is the wrong one. A mailer folds. That is acceptable for a tee shirt and wrong for a certificate or a photo that must not crease.
Packaging Suppliers lists Board-back envelopes separately because the jobs diverge as soon as the goods are on the bench.
What the right line actually does
A board-back envelope keeps a document flat.
Board-back envelopes are a separate list from plain envelopes and from mailing bags.
If you have already mixed them
Folding a board-back job into a mailer because the shelf was empty creases the job.
A Packaging Suppliers basket with Board-back envelopes still leaves the same day if it is placed by 3pm. The terms aim for 5.30pm the next working day. A 1 to 3 day courier is also described, depending on the lines.
Open Board-back envelopes when you already know this line. The pack count is on the product. Packaging Suppliers will still dispatch an order placed by 3pm on the same day.