The swap that looks close
a mailing bag is the usual substitute for plain envelopes, and it is the wrong one. A mailing bag is a parcel outer. An envelope is not a small mailer.
Packaging Suppliers lists Plain envelopes separately because the jobs diverge as soon as the goods are on the bench.
What the right line actually does
Plain and window envelopes are for paper that can go as a letter, not for a parcel.
They sit in papers and envelopes, apart from mailing bags and board-backs.
If you have already mixed them
Putting a thick item in a paper envelope splits it.
A Friday order on Packaging Suppliers that includes Plain envelopes aims for Monday. A bank holiday on Friday or Monday moves that delivery. The 3pm cutoff is still the same-day dispatch rule.
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.