Always pay from the official Dart Charge website. There are fake websites out there that look real. They’ll charge you extra and do nothing. Always check the URL:
gov.uk/pay-dart-charge is your friend. If the URL looks dodgy, run away. Your bank account will thank you.
Also? Pay immediately after you stop the car. Park at a services, pull out your phone, and pay. It takes 30 seconds. Future you will be so grateful.