カスタム投稿タイプのページタイトルを取得したドロップダウンを作リたいです。

<?php

function dequeue_devicepx() {
wp_dequeue_script( 'devicepx' );
}
add_action( 'wp_enqueue_scripts', 'dequeue_devicepx', 20 );
add_action('init','add_categories_for_pages'); 
function add_categories_for_pages(){ 
register_taxonomy_for_object_type('category', 'page'); 
} 
add_action( 'pre_get_posts', 'nobita_merge_page_categories_at_category_archive' ); 
function nobita_merge_page_categories_at_category_archive( $query ) { 
if ( $query->is_category== true && $query->is_main_query() ) { 
$query->set('post_type', array( 'post', 'page', 'nav_menu_item')); 
} 
} 

add_filter( 'arexworks_get_global_loop_variable', 'harman_set_global_loop_variable' );

if(!function_exists('harman_set_global_loop_variable')){
	function harman_set_global_loop_variable(){
		return 'harman_loop';
	}
}

/*
 * Load Frameworks
 */
require_once get_template_directory() . '/framework/load.php';

/**
 * Theme support & Theme setup
 */
if ( ! isset( $content_width ) ) {
	$content_width = 1170;
}

add_action('after_setup_theme', 'harman_after_setup_theme');

if(!function_exists('harman_after_setup_theme')){
	function harman_after_setup_theme(){
		/**
		 * Load Text domain
		 */
		load_theme_textdomain( 'harman', get_template_directory() . '/languages' );
		load_child_theme_textdomain( 'harman', get_stylesheet_directory() . '/languages' );

		/**
		 * Add theme support
		 */
		add_theme_support( 'woocommerce');
		if(defined('WOOCOMMERCE_VERSION')){
			if(version_compare( WOOCOMMERCE_VERSION, "2.1" ) >= 0){
				add_filter( 'woocommerce_enqueue_styles', '__return_empty_array' );
			} else {
				define( 'WOOCOMMERCE_USE_CSS', false );
			}
		}

		add_theme_support( 'automatic-feed-links' );
		add_theme_support( 'title-tag' );
		add_theme_support( 'custom-header' );
		add_theme_support( 'custom-background' );
		add_theme_support( 'post-thumbnails' );
		add_theme_support( 'post-formats', array(
			'quote',
			'image',
			'video',
			'link',
			'audio',
			'gallery'
		) );

		add_theme_support( 'menus' );
		register_nav_menus( array(
			'account-nav'    => esc_attr__( 'Account Navigation', 'harman' ),
			'main-nav'       => esc_attr__( 'Main Navigation', 'harman' ),
		) );

		add_editor_style( 'editor-style.css' );
	}
}

add_filter('harman_filter_show_page_title', '__return_false');

function section_feed_shortcode( $atts ) {
extract( shortcode_atts( array( 'limit' => -1, 'type' => 'post'), $atts ) );
$paged = get_query_var('paged') ? get_query_var('paged') : 1;  
query_posts(  array (
    'posts_per_page' => $limit,
    'post_type' => $type,
    'order' => 'ASC',
    'orderby' =>'menu_order',
    'paged' => $paged ) );
$list = ' ';   
while ( have_posts() ) { the_post();
    $list .= '<article class="listing-view clearfix">'
    . '<div class="listing-content">'
    . '<h3><a href="' . get_permalink() . '">' . get_the_title() . '</a></h3>'
    .'<p>' . get_the_excerpt() . '</p>'
    . '<a href="' . get_permalink() . '">' . '続きを読む &raquo;' . '</a>'
    . '</div>'
    . '<a class="listing-thumb" href="' . get_permalink() . '">' . get_the_post_thumbnail($page->ID, 'listing-thumb')  . '<span></span></a>'
    . '</article>';
}
return
'<div class="listings clearfix">'
. $list
. '<div class="nav-previous">' . get_next_posts_link( __( '<span class="meta-nav">&larr;</span> Older posts' ) ) . '</div>'
. '<div class="nav-next">' . get_previous_posts_link( __( 'Newer posts <span class="meta-nav">&rarr;</span>' ) ) . '</div>'
. '</div>' .
wp_reset_query();
}
add_shortcode( 'feed', 'section_feed_shortcode' );

/* ポートフォリオ選択肢の表示 */
add_action( 'woocommerce_after_order_notes', 'my_custom_checkout_field' );
function my_custom_checkout_field( $checkout ) {

	echo '<div id="portfolio_title_field"';
	
    woocommerce_form_field('portfolio_title', array(
        	'type' => 'select',
        	'label' => 'ご依頼のデザイナーをお選びください',
        	'options' => get_portfolio()
		),
		$checkout->get_value( 'portfolio_title' )
		
	);

	echo '</div>';
}

/* ポートフォリオの選択チェック */
add_action('woocommerce_checkout_process', 'my_custom_checkout_field_process');
function my_custom_checkout_field_process() {
    global $woocommerce;
    if (!isset($_POST['portfolio_title']) ||  empty($_POST['portfolio_title'])) {
    	wc_add_notice('ご依頼のデザイナーを選択してください。', 'error');
    }
}

/* 選択されているポートフォリオタイトルを保存 */
add_action('woocommerce_checkout_update_order_meta', 'my_custom_checkout_field_update_order_meta');
function my_custom_checkout_field_update_order_meta( $order_id ) {
	if ($_POST['portfolio_title']) update_post_meta( $order_id, 'portfolio_title', esc_attr($_POST['portfolio_title']));
}

/* 購入されたポートフォリオタイトルを表示 */
add_action( 'woocommerce_admin_order_data_after_billing_address', 'my_custom_checkout_field_display_admin_order_meta', 10, 1 );

function my_custom_checkout_field_display_admin_order_meta($order){
     echo '<p><strong>ご依頼のデザイナー:</strong> ' . get_post_meta($order->post->ID, 'portfolio_title', true) . '</p>';
}

/* メールに追加 */

add_filter( 'woocommerce_email_order_meta_fields', 'custom_woocommerce_email_order_meta_fields', 10, 3 );
function custom_woocommerce_email_order_meta_fields( $fields, $sent_to_admin, $order ) {
    $fields['meta_key'] = array(
        'label' => __( 'ご依頼のデザイナー' ),
        'value' => get_post_meta($order->post->ID, 'portfolio_title', true),
    );
    return $fields;
}

/* ポートフォリオ選択肢の生成 */
function get_portfolio(){
	$portfolio = array('' => '');
	$args = array(
		'post_type' => 'portfolio',
		'numberposts' => 0
	);
	$posts_array = get_posts( $args );
	$size = count( $posts_array );
	for( $i=0 ; $i < $size ; $i++ ){
		$portfolio += array($posts_array[$i]->post_title => $posts_array[$i]->post_title);
	}
	wp_reset_postdata();
	
	return $portfolio;
}

適当にやって見たのですが、メールの画面に関しては

/* メールに追加 */

add_filter( 'woocommerce_email_order_meta_fields', 'custom_woocommerce_email_order_meta_fields', 10, 3 );
function custom_woocommerce_email_order_meta_fields( $fields, $sent_to_admin, $order ) {
    $fields['meta_key'] = array(
        'label' => __( 'ご依頼のデザイナー' ),
        'value' => get_post_meta($order->post->ID, 'portfolio_title', true),
    );
    return $fields;
}

で追加することはできました。