Note: You CANNOT refund an order via Shopify, Bigcommerce or Omnivore as all payments are handled by the Market, not your website or Omnivore.
Unfortunately, the Market orders cannot be refunded via Omnivore.
To refund an order, you must do so by logging into your the Market seller account.
For instructions on how to cancel an order, and refund the customer please see go to your the Market seller account
To prevent further notifications about the order you should then mark the order as refunded in Omnivore from the order page. In Omnivore go to Orders->All Orders and search for the order. Once you have found the order click on View to the right of the order then "Mark as Refunded"