How do I (partially) refund customers in case I had to, or they had to cancel the participation in a trackday?

You can do the (partial) refund in two ways:

1. Use the Stripe functions by logging into your own Stripe account, search for the corresponding booking(s) and use Stripe’s own refund function. This refund function allows you to refund the client fully or partially for their booking. Stripe will automatically do the refund to the client’s payment method (i.e. refund on credit card or bank account).
2. You can ask the customer directly for their bank account details and wire the amount due back into their bank account