As SlackAlice says, it's always been made clear that you have to pay at a PayZone facility (e.g. in many supermarkets or convenience stores), online or via an account. The crossing is used by around 130,000 vehicles per day; they're not going to write to every driver for payment!
Even if you only use the Dartford Crossing once or twice per year (or just think that you might do), it's easiest to set up an account, so that the charge is automatically paid from your bank account:
https://www.gov.uk/pay-dartford-crossing-charge
All you can do now is to join the millions of others who've had to pay up:
http://www.bbc.co.uk/news/uk-england-kent-34968025