MIGS Email notification

Hi,

You can edit the file plugins/hikashoppayment/migsvpc/migsvpc.php and change the line:

$email->subject = JText::sprintf('PAYMENT_NOTIFICATION_FOR_ORDER', 'MIGS', $payment_status, $dbOrder->order_number);

As you can see in the line of code I gave you, which comes from the MIGS payment plugin, the order number is already included in the subject of the payment notification. So you shouldn't have to do anything, unless your translation of that PAYMENT_NOTIFICATION_FOR_ORDER translation key doesn't include the parameter for the order number display.
So I would recommend to check that first.