In the plugin configuration, you have the option of setting absolute amounts as reminder fees for each of the three reminder levels. These are added up over the different levels. You can find the fees in the tab Overview in the free text field Dunning fees (Mahngebühren). However, the open invoice amount is not increased by the reminder fees in the order overview.
In Germany, the charging of reminder fees and their amount is subject to legal regulations (in particular §286 and §288 BGB). Please inform yourself about the corresponding regulations in your country.