You should advise passengers of the alternative routes available, and the likely costs and times associated with each. The customer is entitled to choose the route, even if you disagree with their choice.
If the chosen route is subject to toll charges, you are entitled to include the toll charge in the fare charged to the passenger. Such toll charges should be recorded on the receipt.
If you are using an electronic tag to pay the toll, it may not be obvious to the passenger what the charge is. You are advised to keep a schedule of toll charges in the car, so that you can show it to customers if necessary (for example, on the M50, or in the Port Tunnel).