Sorry you had a bad experience - the driver probably noticed your foreign number/account.
There are no premium or luggage fees, so you can decline. If they still add it manually to your bill, grab will refund. The manual add-on exists for tolls etc, so I always watch them end the ride without adding anything before I get out of the grab car...
Not sure how the interface looks from the driver view, but I've not experienced haggling problems in Malaysia, Thailand, Singapore etc on Grab.
Normally I would just move on to the next taxi / ride if I'm not comfortable, however the crowds at Terminal 2 at the time just made it seem even more of a hassle.