priscilla,
I can understand the confusion since you provide three levels of service. As a suggestion you can either provide whatever service you have time to provide, or ask the pilot what level he or she desires.
Here in the US if a VFR pilot calls and ask for VFR flight following and traffic permits we will provide radar service (flight following) and issue known traffic. If we are too busy we tell the pilot unable and to stay clear of the Class C (Charlie) airspace.
Mike
NATCA FWA