// The following example is for adding a hook callback.
// define the woocommerce_calculated_total callback
function filter_woocommerce_calculated_total( $round, $instance ) {
// make filter magic happen here...
return $round;
};
// add the filter
add_filter( 'woocommerce_calculated_total', 'filter_woocommerce_calculated_total', 10, 2 );