add_filter('woocommerce_cart_calculate_fees', 'add_additional_shipping_fee');
function add_additional_shipping_fee() {
    $shipping_country = WC()->customer->get_shipping_country();

    if ($shipping_country == 'US') {
        WC()->cart->add_fee('Zusätzliche Versandgebühr', 10.00);
    }
}