I don't really like what this method signature has become.

What if you extracted

```
usage_minutes = (act_end_at - act_start_at) / 60
discount = calculate_discount act_start_at, act_end_at
cost_and_subsidy usage_minutes, discount
```
to a new method that takes two times. Then in `calculate_overage` it adds the calculate_reservation price to any overrage time (calculate the start and end of the overrage period inside `calculate_overage`.