/**
  * Create refund on stripe servers
  *
  * @access      public
  * @param       string $transaction_id
  * @param       array $refund_data
  * @return      array
  */
 public static function create_refund($transaction_id, $refund_data)
 {
     return S4WC_API::post_data($refund_data, 'charges/' . $transaction_id . '/refunds');
 }