You can cancel the payment as a merchant by yourself in the ePassi online service within an hour. 

Log in to the online service with your user ID. Choose “Transactions,” which takes you to the payment transaction page. Once you’ve found the incorrect payment, click on Cancel. The amount will be canceled and returned to the customer's ePassi account.

If the incorrect payment was made more than an hour ago, you can cancel the charge by sending an email to info@epassi.fi. In the email, tell the date and time of the payment, the sum, and the customer’s name and phone number. When the amount has been canceled, you’ll get a notification.