When refunding advances via Mollie, I also want the option not to refund the full amount, but the amount minus the Mollie transaction costs + refund costs.

Example:

iDEAL €100 → customer gets €99.46 back

Credit card €100 → customer gets €97.70 back

Legal/practical

I cannot simply deduct costs; in principle, my customer is entitled to a full refund in case of withdrawal (within 14 days, consumer purchase).

So optionally, only when it comes to a different situation (e.g. voluntary leniency refund), you can charge costs under certain conditions.