If the karwa drivers demands more, agree and board the taxi, take down the taxi details and possible take a picture of the driver, when you reach the destination get down from the taxi pay the meter amount and walk away.
If he has not switched on the meter then ride is free, driver will try to argue that's when you pay him the minimum amount and tell him you will complain Karwa immediately.
NOte always take the back seat and remember you can always open the door even if it is locked by drivers, use the knob in the door handle. I am regular karwa user and it has worked each time.
Some times driver ask for destination before boarding and might not pick you if it is far off place or if the destination has traffic jam, all you have to do is board the taxi telling you will pay extra, again once you reach place of destination, just pay the meter amount and walk away.