Plain envelopes are letters. They are not small parcels and they are not a cheap mailer. This page stays with plain envelopes.
Packaging Suppliers keeps Plain envelopes apart from a mailing bag. They sit in papers and envelopes, apart from mailing bags and board-backs.
What this collects
Plain and window envelopes are for paper that can go as a letter, not for a parcel.
The notes below stay on plain envelopes. A mailing bag is a parcel outer. An envelope is not a small mailer.
Before the order goes in
Window or plain is the choice, then the paper size.
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.