The job it is listed for
A mailroom running the shelf down starts from the goods. Plain and window envelopes are for paper that can go as a letter, not for a parcel. Packaging Suppliers keeps Plain envelopes as its own range so it is not picked as a mailing bag.
They sit in papers and envelopes, apart from mailing bags and board-backs.
The line that is not a substitute
A mailing bag is a parcel outer. An envelope is not a small mailer.
Window or plain is the choice, then the paper size.
Bringing it in
Letter post and parcel post run out on different days.
At Packaging Suppliers, Plain envelopes does not change the cutoff. Orders by 3pm leave the same day. Under £50 ex VAT the delivery is still available. It is just not the free threshold.
Open Plain envelopes when you already know this line. Boxes of envelopes are the unit. Read the count. Packaging Suppliers will still dispatch an order placed by 3pm on the same day.