I had at least 3 occasions where Itrium Justitia (debt collection agency) tried to charge me "processing costs", normally, like extra 1/3 of the debt amount. My legal insurance directed me to this page that contains an example letter, basically saying "I'm happy to pay the debt and interest, but charging processing costs is illegal, go away!"
Works like a charm
https://www.konsumentenschutz.ch/the...t-musterbrief/