The lines that belong together
Order board-backs for the flat post and mailers for the soft post.
They do not need a carton unless you are sending many in one outer.
The line that stays separate
a folded mailer stays on its own order line. A mailer folds. That is acceptable for a tee shirt and wrong for a certificate or a photo that must not crease.
Board-back envelopes is the range Packaging Suppliers uses for board-back envelopes.
One basket
Board-back envelopes are a separate list from plain envelopes and from mailing bags.
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.