How to handle refunds / returning goods with drupal commerce
There hasn't been created any module that handles this. How to approach this also depends a lot on the custom logic in your store like


How are payments created
How should payments be refunded
What happens if returned goods has damage
...


Anyways, what you could do for starters, is to create a custom order status to mark that the goods have been returned. Most of the rest is most likely custom store logic anyways.