This is my experience from 25 years ago in The Netherlands:
I was caught by a Gatso camera by Schipol airport as I was rushing to make my flight on a Friday afternoon. A few weeks later, I received a letter from the Ducth courts indicating a fine of around €50 IIRC. I ignored this letter and over the course of the next few months, I received a couple more letters, each escalating the fine. I also ignored these letters and thought no more of it.
3 years later, I next visited The Netherlands (for a stag do) and upon presenting my passport at the immigration desk, I was taken aside and very politely interrogated about the events of 3-years previously. I denied all knowledge, but reluctantly accepted that unless I paid the €99 owning, I would continue to be detained. Everything was handled politely and in a friendly tone, so I was not traumatised at all, but I learned a lesson.
The hire company had my passport details as well as my driving licence details and passed these on to the courts. The courts were able to flag my passport and hence I was prevented from entering the country until the matter was resolved.
For peace of mind, I would pay at the earliest opportunity. I suspect it is possibly nowadays to pay online rather than trying to send a money-order etc
