Gst for service of vehicles

Dear Forum, When a vehicle is serviced in a Authorized Service Station which GST rate is to be charged. GST on Service (labour) is 18% and GST on Sale of Spare parts is 24%. Here Prinpicle supply is of service as it is a authorized service station. 1) So should the Authorized Service Center apply 18% as being principle work is supply of service on whole value instead of 24%. 2) Or should they charge 24% being highest rate of GST on sale of Spares. 3) Should they make two separate bills for sale of service and sale of spares and do all the paper work extra. 4) Or should they make a single bill and charge 18% on supply of Service and 24% on Supply or spares.