When creating dunning flows and setting up different dunning flow levels, you have the possibility to apply dunning fees to the invoice for each dunning flow level:
In the dunning flow's configuration, you can define whether tax should be applied to the dunning fee(s) or not:
Make sure to check if the dunning fees are subject to VAT in your jurisdiction. According to the Swiss Federal Tax Administration, the dunning fees are not subject to VAT.
You should therefore select 'Apply no Tax' as you can see above.