选择列表更改时更新结帐总额

How i can update the checkout total on selection list change event. I am using ajax to update the price. Code:

add_action("wp_ajax_my_downpay", "my_downpay");
add_action('wp_ajax_nopriv_my_downpay', 'my_downpay');

function my_downpay() {
    global $woocommerce;

    $woocommerce->cart->add_fee( 'Ozdobne pudełka:', 20 );
    die();
}

Selection list screen short: http://prntscr.com/lay65b