You’re on the right track, but have the wrong specifics.

A merchant accepting card payments might pay “sticker rate” for processing, 3%, at the outset; but, they eventually get IC+ pricing as their volume increases. This might be closer to 1-2%.

If they pay 5% to a BNPL, that extra 2-4% is meant to cover the interest of the “loan” if the customer doesn’t pay back on schedule.