Custom counter for Toll Tax and Maintenanc expens calculation

I need to calculate the toll tax paid by my vehicles using geofence notifications.
I have created geofences at the toll gate locations. I want to have a custom parameter which will be increased by a certain value when the vehicle crosses a particular geofence. I wish to do this using geofence notification.

Let us say, we have a custom parameter in unit properties called - "toll". We set the initial value of "toll" as ZERO or any other value we like. (exactly the same way we set the values of mileage counter or GPRS traffic counter at the of installation of tracker).

Now, when a vehicle crosses Geofence - A, the "toll" should be increased by 30 - i.e. new value of "toll" becomes 30.

Next, the vehicle passes through Geofence - B, the "toll" is increased by 50 - i.e. new value of "toll" becomes 80.

And then we can generate all kinds of reports on toll tax paid - for any unit or unit groups and for any interval of time. Also, the unit tooltip will show the current cumulative value of "toll" - lie it shows the value of mileage counter.

Such custom parameters can be used in many different ways. Such as - Routine maintenance. For example OIl change almost always cost the same and is done at fixed intervals. This kind of custom variable will help up make a counter of the amount spent on a certain type of service.