As of October 2014 payment will no longer be accepted at booths in cash.
New payment methods are similar to those used to collect payment for the London Congestion Charge.
Drivers will no longer pay at the barriers; instead you’ll be able to pay in advance, or before midnight the day after crossing. Drivers who don’t pay will face a penalty charge.
The new scheme is known as Dart Charge and will help to reduce congestion at the crossing.
It will still be free to use the crossing between 10pm and 6am.
Further details can be found here