Enable free shipping for registered users

add_filter('woocommerce_package_rates', 'free_shipping_for_logged_in_users', 10, 2);
function free_shipping_for_logged_in_users($rates, $package) {
    if (is_user_logged_in()) {
        foreach ($rates as $rate_id => $rate) {
            if ('free_shipping' === $rate->method_id) {
                $rates[$rate_id]->cost = 0;
                break;
            }
        }
    }

    return $rates;
}