We use a Motorvac machine at our shop. you hook it up to the fuel lines and it runs off the additive rather than the tank fuel. not sure what they are selling you but this machine also cleans the valves to some extent as well as the combustion chamber. We charge $150 CDN here. I'd probably start with the basics though. Tire pressures, tune up. O2 sensors also affect fuel economy. It is a decent service IMO.
Mark