Open the order → Make a return label. It's bought on your own EasyPost or Shippo account at the cheapest service, from the customer's address back to your location.
The customer gets Your return label from your store's email, with a Print your return label button. The label also shows on the order with its tracking number, and you can void it if it isn't used.
Refund the order when the package comes back (see Refunds and cancels).