There is a message template called "Additional Payment Receipt or Refund Notification" in the System workflow messages. I have tried to trigger the Refund Notification by adding contributions and then setting their status to be "Refunded" and I thought this would trigger the email notification, but it didn't. I have tried with event registrations too, but got no email. Am I doing something wrong, or did I miss a step? I couldn't find any information about the refund email notification in the CiviCRM guide either. I have tried on 4.6.33 and 4.7.27 installations but doesn't seem to work on any of them.
Let me know please if you have the answer. Thank you!