add_action('add_meta_boxes', 'kargo_takip');
function kargo_takip() {
    add_meta_box('kargo_takip_meta_box', 'Kargo Takip', 'kargo_takip_meta_box_ekle', 'shop_order', 'side', 'high');
}

function kargo_takip_meta_box_ekle() {
    global $post;

    $meta_field_data = get_post_meta($post->ID, '_kargo_takip', true) ? get_post_meta($post->ID, '_kargo_takip', true) : '';

    echo '
        <input type="hidden" name="kargo_takip" value="' . wp_create_nonce() . '">
        <p style="border-bottom:solid 1px #eee;padding-bottom:13px;">
        <input type="text" style="width:250px;";" name="kargo_takibi" placeholder="' . $meta_field_data . '" value="' . $meta_field_data . '"></p>
        ';
}

add_action('save_post', 'kargo_takip_kaydet', 10, 1);

function kargo_takip_kaydet($post_id) {

    if (!isset($_POST['kargo_takip'])) {
        return $post_id;
    }
    $nonce = $_REQUEST['kargo_takip'];

    if (!wp_verify_nonce($nonce)) {
        return $post_id;
    }

    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) {
        return $post_id;
    }

    if ('page' == $_POST['post_type']) {

        if (!current_user_can('edit_page', $post_id)) {
            return $post_id;
        }
    } else {

        if (!current_user_can('edit_post', $post_id)) {
            return $post_id;
        }
    }

    $kargo_takip_no = $_POST['kargo_takibi'];

    $order     = wc_get_order($order_id);
    $useremail = $order->get_billing_email();

    update_post_meta($post_id, '_kargo_takip', $kargo_takip_no);
    wp_mail($useremail, "Your order picked up", "Your track code: " . $kargo_takip_no);
}