Service/Support Tracking

Service/Support Tracking

I need some help here.

My organization sells capital equipment and much of our equipment includes a set amount of support time as well as a warranty. We also provide support outside of warranty at $95/hour.  I planned on having our admins create a case which would then be approved and distributed to the appropriate service technician. 

To complicate things, we bill a minimum of 2 hours. If the customer only uses 45 minutes the first time they call in, they'd have a balance of an hour and 15 minutes. Each time they call we'd deduct from this until it reach zero at which point we'd bill for another 2 hours. 

Does anyone have a good way of tracking this?