$('.wcd-cart-wrapper input[name="wcd_option"]').on('change', function() {
    var data = {
        'enabled': $(this).val(),
        'action': 'update_cart_subtotal'
    };

    $.post( ajaxurl, data, function( response ) {
            console.log( response );
    });
});

public static function update_cart_subtotal() {
    $enabled = $_POST['enabled'];
    $value = 20;
    $cart = WC()->cart;
    if( 'enabled' == $enabled ) {
        $cart->set_subtotal($value);
    }

    wp_send_json( $cart->get_subtotal() );
}