The Dartford Crossing is a tolled bridge & tunnel in East London, which has a small automatic toll charge to cross. This is often called the 'Dart Charge' and applies by reading the vehicle license plate.
Both the Dartford Charge and the London Congestion charge are included in your reservation cost, so there's no need for concern or payment. The license plate will be recognised as a Zipcar vehicle and no charge will be incurred.
If you have paid the Dartford Charge for a Zipcar in error, you can contact them via the following link: