When you are taking the free bus from one airport to the other, you don't just hop on the bus when it pulls up. You need to show your ticket to someone sitting at a table who puts you on the list of who can ride the bus. Although I've ridden the bus at least a half dozen times, I don't remember if they stamp your hand or what, but there is something that designates that you are eligible to ride the bus. The first time I rode the bus was from DMK to BKK. I showed my itinerary on my phone, but it didn't meet their qualification. I wound up having to go to a desk on the first floor of DMK where they called the airline to confirm I was on the the flight. It's not something they seem to take lightly. Since then, I've always had my actual BP when possible.