code: def refund_order(self, order_id, **params):
          
          for required in ['currency']:
              if required not in params:
                  raise ValueError("Missing required parameter: %s" % required)
          response = self._post('v2', 'orders', order_id, 'refund', data=params)
          return self._make_api_object(response, Order)
docs: https://developers.coinbase.com/api/v2#refund-an-order