Count the job, not the habit
Letter post and parcel post run out on different days.
Count letters.
Where it lives during that week
Keep window and plain in separate trays.
Order envelopes on the letter line. Do not hide them in the mailer quantity.
The reorder line
Say plain or window, and the 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.
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.