I have a 2003 mercedes E500

mbusa.com has a site to explain and register your car.

The "C" service isn't real. It's something that dealers put together to get cash business and will naturally vary from dealer to dealer.
The FSS system only goes "A,B" there is no C.

Don't misinterpret that I'm saying your car does not need a tune-up, just be wary of what the service includes.

This is where dealer trust often goes sour as the bill will have some questionable things listed on the repair order that are sometimes not actually carried out.

Things like "brake flush", "engine flush", fuel/oil additives come to mind and some dealers have the nerve to include shop supplies.

They will also use the time in the shop to sell you other "gravy jobs" like tires and brakes.

My advice is skip the C service, get an oil change.

Your car is at an age where you as the owner need to be smart where your money is spent.

If your not the type of person that knows the basics on their own whether your brake fluid is too old or coolant needs to be changed, brakes tires etc..then I recommend getting into a new car every few years. The car business is set up to make life easier to do this and there is nothing wrong with doing just that.

That's not just MB advice, that last part goes for any car your driving.