How about instead you discussu a SLA (Service Level Agreement). For example you agree that emails / calls are to be acknowledged or responded within 1 hour during 9-5 Mon-Fri for £/$XXX per month.
You could go a step further any have different levels, so for a higher monthly rate you'd respond within 30 minutes, etc etc.
You could go a step further any have different levels, so for a higher monthly rate you'd respond within 30 minutes, etc etc.