title: Add custom calculation to cart total and grand total in magento
body: <p>I am working on site where i want to add/subtract fee to cart total and grand total.I am firing this event to capture the cart details.<code>sales_order_save_after</code>. while in observer i got the price using this code </p>
  
  <pre><code>public function modifyPrice(Varien_Event_Observer $obs)
          {
  
  
         $getotal = Mage::helper('checkout')-&gt;getQuote()-&gt;getGrandTotal();
         $totals = Mage::getSingleton('checkout/session')-&gt;getQuote()-&gt;getTotals(); 
         $subtotal = $totals["subtotal"]-&gt;getValue();
  }.
  </code></pre>
  
  <p>But i don't know how to add/subtract amount from total and than update accordingly.Thanks in advance</p>