Might be legit. If your car is newer than 2006, then it likely has direct injection. Because vehicles with direct injection do not have any gas in the intake, carbon from the EGR and PVC systems can build up on the tops of the valves and the lower valve stems, slowly reducing the cross section for air to enter the engine, slowly choking the engine to death. Eventually it will lead to a burnt valve.
There are new treatments for this but it is not as simple as adding an additive to the gas, either in the tank or at the fuel rail. It has to be sprayed through an adapter directly into the intake at the throttle body and it is not a quick procedure. It will take all of an hour of the mechanics time so doing this for $100 at a dealership does seem very unlikely, considering that the chemicals alone cost about $40.
Talk with the dealership service rep and ask for a description. If he says that they hook up an adapter to the fuel injection system to clean out the injectors, that is a legitimate service but will do you little good. It is mainly for pre 2006 engines with port injection and is likely outdated for your vehicle. They need to catch up with the times.