Ask him to show where in the owners' manual "Throttle service" is mentioned or required? If your car has been exclusively run on petrol or giving no trouble in engine idling or starting etc. then this so called "Throttle service" is absolutely a big "No." I've many stories from friends where a car which was running perfectly ok started giving trouble after a Toyota 3S "Throttle service."
What are "inspection" charges? Are these for just visually checking brake oil, battery electrolyte, engine coolant levels or drive belt condition etc. then probably you yourself can do it better than "chhotey ustaads" at 3S.