As some of you will know, the Dartford crossing toll can now only be paid online, instead of throwing money into a bucket. It has to be paid by midnight on the following day. There is a numberplate recognition system in operation. There are signs telling you this, but they are not very clear, and only in English. I used the crossing in August.
I didn't pay, not because I wanted to avoid payment, but because being on holiday, I didn't have access to a computer, and I'm not so daft as to enter credit card details into an internet café computer, even if I could be bothered to find an internet café. And I wasn't going to be home for 3 weeks.
In December I got a letter, in good German, requiring me to pay an excess charge of about 100 euros, and a threat to take me to court in Germany. However, 'as a foreigner, I might not be aware of the system', so they would give me the chance to pay the original toll without the penalty. Which I did. I also sent them a mail containing roughly the second paragraph above, and saying that I didn't rate their chances of winning their case in the German courts as very high. They replied acknowledging payment, but not responding to the points I'd made.
My conclusion is that you can wait until you get home to pay. (I wouldn't take the threat of legal action to claim the penalty too seriously.)
(I once had a demand from a debt collector in England for a parking fine. But as it was written in atrocious German, didn't say what it was that I was alleged to have done, was two years or more after the alleged event, and used language suggesting they were a public authority, I just said 'See you in court, where I will charge you with Amtsanmaßung.' And that was the last I heard.)