Fine in train in Netherlands?

Last month I was travelling by train in Netherlands and I bought a ticket with the discount card, but during the control they found out that the discount card was already expired. They want to charge me 28 Euros for the full price ticket and 35 Euros fine, they told me that they would send me a letter but I haven't received anything yet. I am leaving Netherlands in a few weeks, I am going to the Czech Republic and I am not planning to come back. If I don't pay my fine will they execute it internationally?