trash fine extortion

Hi

I posted about a trash fine last week. Thanks for all the replies.
Basically the fine was 75 euros 6 months ago. We paid it on time.
Today they told us to pay another 75 euro for "admin costs". So in total we have to pay 150 euro for putting some paper in the general waste bin. This happened just after we moved to Brussels from another commune and was our first "trash offence" in 10 years of living in Belgium. We made the mistake because the rules are different to our previous commune.
We spoke to them over the phone, but it seems they are insisting on us paying for it.
This seems so unreasonable to us .. has anybody else had this experience ?