I need to modify shipping prices for a particular product, while maintaining my weight table for all other products.

I've got the following observer working, but I'm a bit at a loss as to what I should be doing in the update function. Is there a way to direct the app to a second rate table for this product? Or should I just modify the shipping cost variables directly? And what is that variable's name?

class customShipping extends base {

var $customShippingID = 37;
function customShipping() {
$_SESSION['cart']->attach($this, array('NOTIFIER_CART_ADD_CART_END', 'NOTIFIER_CART_REMOVE_END'));

}

function update(&$class, $eventID) {

if ($_SESSION['cart']->in_cart($this->customShippingID)) {
$myquan=$_SESSION['cart']->contents[$this->customShippingID]['qty'] ;
$shipping = 5*$myquan ;
}
}
}