Measure before you open the range
Window or plain is the choice, then the paper size.
Plain and window envelopes are for paper that can go as a letter, not for a parcel. The page for Plain envelopes is where Packaging Suppliers shows that line.
The published limit, if there is one
They sit in papers and envelopes, apart from mailing bags and board-backs.
A mailing bag is a parcel outer. An envelope is not a small mailer.
Write the size down
Say plain or window, and the size.
On Packaging Suppliers, orders that include Plain envelopes and are placed by 3pm are dispatched the same day. UK delivery is free on orders over £50 ex VAT. Stocked items can be returned within 14 days.
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.