The swap that looks close
a padded envelope is the usual substitute for grey mailing bags, and it is the wrong one. A padded envelope is the lined bag. A grey mailer does not add that cushion.
Packaging Suppliers lists Mailing bags separately because the jobs diverge as soon as the goods are on the bench.
What the right line actually does
Grey mailing bags are the opaque outer for soft goods.
They are also called shoe bags. The category says they are European made from recycled film and 100% recyclable, and opaque so the contents are not visible.
If you have already mixed them
Dropping a fragile item into a grey mailer because the padded envelopes are out removes the cushion.
At Packaging Suppliers, Mailing bags 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 Mailing bags when you already know this line. Packs are the unit. Read how many bags are in the pack. Packaging Suppliers will still dispatch an order placed by 3pm on the same day.