The job it is listed for
A mailroom running the shelf down starts from the goods. A board-back envelope keeps a document flat. Packaging Suppliers keeps Board-back envelopes as its own range so it is not picked as a folded mailer.
Board-back envelopes are a separate list from plain envelopes and from mailing bags.
The line that is not a substitute
A mailer folds. That is acceptable for a tee shirt and wrong for a certificate or a photo that must not crease.
Pick the envelope from the document size, not from the nearest mailer.
Bringing it in
The flat-document slot runs out on its own. Mailers can still look plentiful.
On Packaging Suppliers, put Board-back envelopes on the order with the lines it belongs to, so you are not paying a second delivery for the piece you forgot. Free UK delivery starts over £50 ex VAT.
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.