The fare is agreed at the point of booking, based on the journey itself, not calculated as the car moves. That means it stays the same if the A6 is slower than usual, if a diversion adds a few extra minutes, or if the journey simply takes longer than planned because of traffic elsewhere on the route. What's quoted at booking is what gets charged at the end, with no adjustment for how the roads behaved on the day.
Every booking includes a fully licensed driver, and cancellations are free if plans change before the pickup. That combination matters for anyone booking well ahead of an important journey, an early flight or a cruise departure, since there's no financial penalty for booking early and then needing to adjust the details later.