How can I change grand total when user Clicks on Custom Payment method?

Question

I am using the one-page checkout module. I want to change the grand total when user/customer selects the payment method. If customer/user selects the online payment option then grand total should change.

app/code/local/Bliss/Onlinepayment/etc/config.xml

<global>
    <events>
        <payment_method_is_active>
            <observers>
                <observer>
                    <type>singleton</type>
                    <class>Bliss_Onlinepayment_Model_Observer</class>
                    <method>paymentmethod</method>
                </observer>
            </observers>
        </payment_method_is_active>
    </events>   
</global>

app/code/local/Bliss/Onlinepayment/Model/Observer.php

<?php

class Bliss_Onlinepayment_Model_Observer {

public function paymentmethod(Varien_Event_Observer $observer) 
    {
        //$method = $observer->getEvent()->getMethodInstance();
        $quote = $observer->getEvent()->getQuote();
        if($quote->getCode()=='checkmo')
        {

   $quote->setGrandTotal(1);
   $quote->setBaseGrandTotal(1);
   $quote->save();

        }


    } 
}