paid the fine on time, but... (enforcers)

I paid the fine on time (255 UAH), and two weeks later a letter arrived from the enforcement service — now they demand 510 UAH and invite me to come in. This has happened to me before, though a long time ago: I came, explained that I paid on time, but still had to pay some small amount at the cash desk. I don't know what exactly for — possibly for the fact that they wrote to me at all. Here's my question: can I just ignore them and not go? I paid everything on time. I admit I violated, I paid the fine, but this is too much. Honestly, the fine itself was made up out of thin air — they scared me into paying. I'm not one to make a fuss, but there's a limit to everything.