Tipping is completely optional, but if your customers want to tip the driver, they can.
The price of the trip includes costs associated with the vehicle, distance and mileage, as well as time spent at any sightseeing stops. Gratuity is excluded from the total price and is not a requirement.
If your clients are extremely satisfied with the service, they can reward the driver with an additional cash tip. 10% is sufficient, however, this may vary according to tipping customs in specific regions.