in success.phtml template you can use
$order = Mage::getModel('sales/order')->loadByIncrementId($this->getOrderId()); $total = $order->getGrandTotal();
The correct way is to extend Mage_Checkout_Block_Onepage_Success
and add your own method to Mage_Checkout_Block_Onepage_Success
order (as on this page, the quote is already inactive), since it is not good to load such material into templates
source share