This is the very first time for me hearing that an airline charges passenger for the baggages they brought up to the plane. I think the next thing which they will charge is the weight of the passenger because
“Lighter baggage on board means burning less fuel, hence, less pollution and a better environment. Fewer checked-in baggage improves airport efficiency and will eventually lead to shorter check-in queues,” he added.
Similarly, lighter passenger should pay less compare to heavier passenger. Lets say a lady weights 60kg with a 30kg baggages and another men weight 120kg with a 5kg baggages, who should consume more fuel?
Anyway, you are suppose to do it 48 hours before your flight. I know someone flying back to Malaysia after 21st April, therefore, I will update the actual process. Currently, you will have to try it out yourself at AirAsia online check in.