I've passed crossing more often since I moved nearby. I always paid online £2.50. Last journey I decided to create account and top up £10 to save time. What a surprise they only charged me £1.67 each way. I think it's worthy to share with others as I couldn'd find this on hukd.
After you create account you have to add your vehicle(s) details and top up 10 pounds (if you decide to close account, you will get remaining credit refunded).
You can choose to auto top up if balance goes below chosen amount or top up manually. Great saving!
It looks like you can top up any amount, I just did £1.69 and i passed (may not apply to initial top up)
link to create account online:dartford-crossing-charge.service.gov.uk/Hom…ype
As mentioned below, it's better to top up initially £10.02 instead of £10 as it will cover 6 crossings.
I opened account for friend, he did 2 crossings over the weekend few months ago and was not charged a penny! We checked his account details and reg. and all correct. He didn't receive penalty either. He visited me again recently and had no charge for 1 of 2 crossings!
Pretty good chance of free crossing, the system isn't working perfectly! If you pay £2.50 on the day of the crossing, you don't know if they managed to record your reg. or not! Let them decide with prepaid account if they want your £ or not