FBM – Partial Refund or Restocking Fee

A shipment was returned to me because the package was undeliverable. Apparently the customer does not have a mailbox. I have reached out to the customer, but they have not responded for days.

I have now received the item back and want to refund the customer less my original shipping cost. How do I handle this? None of the partial refund options match my issue and I want to refund greater than 50% of the order?