Ordenar posts por tipos de post personalizados en un botón select

Buenas!

En el menu de administración de WordPress tengo creados varios «custom-posts» cada uno con sus posts. Ahora en uno de esos post personalizados he creado un campo que recupera todos los posts de los posts personalizados. El problema es que necesitaría que los ordenara de la siguiente manera:

Post Personalizado 1:
Posts
Post Personalizado 2:
Posts
…

Os paso el código que crea el campo y recupera y guarda los datos de select:

// Add the Meta Box  

function add_custom_meta_box_related() {
    add_meta_box(
        'custom_meta_box_related', // $id
        'Related Information', // $title
        'show_custom_meta_box_related', // $callback
        'related', // $page
        'normal', // $context
        'high'); // $priority
}
add_action('add_meta_boxes', 'add_custom_meta_box_related');

    // Field Array
    $prefix_related = 'custom_';
    $custom_meta_fields_related = array(  

		array(
			'label' => 'Related Items',
			'desc' => 'Select a related item(s)',
			'id'    =>  $prefix_materiales.'post_id',
			'type' => 'post_list',
			'post_type' => array('products','paso1','paso2','paso3','paso4','compra'),
)
    );  

	// The Callback
function show_custom_meta_box_related() {
global $custom_meta_fields_related, $post;
// Use nonce for verification
echo '<input type="hidden" name="custom_meta_box_nonce" value="'.wp_create_nonce(basename(__FILE__)).'" />';  

    // Begin the field table and loop
    echo '<table class="form-table">';
    foreach ($custom_meta_fields_related as $field_related) {
        // get value of this field if it exists for this post
        $meta_related = get_post_meta($post->ID, $field_related['id'], true);
        // begin a table row with
        echo '<tr>
                <th><label for="'.$field_related['id'].'">'.$field_related['label'].'</label></th>
                <td>';
                switch($field_related['type']) {
                    // case items will go here 

					// post_list
					case 'post_list':
					$items = get_posts( array (
						'post_type' => $field_related['post_type'],
						'posts_per_page' => -1
					));
					echo '<select multiple name="'.$field_related['id'].'" id="'.$field_related['id'].'">
					<option value="">Select One or more</option>'; // Select One
					foreach($items as $item) {
						echo '<option value="'.$item->ID.'"',$meta_related == $item->ID ? ' selected="selected"' : '','> '.$item->post_title.'</option>';
					} // end foreach
					echo '</select><br /><span class="description">'.$field_related['desc'].'</span>';
					break;
                } //end switch
        echo '</td></tr>';
    } // end foreach
    echo '</table>'; // end table
}

// Save the Data
function save_custom_meta_related($post_id) {
    global $custom_meta_fields_related;  

    // verify nonce
    if (!wp_verify_nonce($_POST['custom_meta_box_nonce'], basename(__FILE__)))
        return $post_id;
    // check autosave
    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE)
        return $post_id;
    // check permissions
    if ('page' == $_POST['post_type']) {
        if (!current_user_can('edit_page', $post_id))
            return $post_id;
        } elseif (!current_user_can('edit_post', $post_id)) {
            return $post_id;
    }  

    // loop through fields and save the data
    foreach ($custom_meta_fields_related as $field_related) {
        $old = get_post_meta($post_id, $field_related['id'], true);
        $new = $_POST[$field_related['id']];
        if ($new && $new != $old) {
            update_post_meta($post_id, $field_related['id'], $new);
        } elseif ('' == $new && $old) {
            delete_post_meta($post_id, $field_related['id'], $old);
        }
    } // end foreach
}
add_action('save_post', 'save_custom_meta_related');

Supongo que en este punto

// post_list
case 'post_list':
$items = get_posts( array (
'post_type' => $field_related['post_type'],'posts_per_page' => -1));
echo '<select multiple name="'.$field_related['id'].'"id="'.$field_related['id'].'">
<option value="">Select One or more</option>'; // Select One
foreach($items as $item) {
echo '<option value="'.$item->ID.'"',$meta_related == $item->ID ? 'selected="selected"' : '','> '.$item->post_title.'</option>';
} // end foreach

tengo que requerir el id de cada post personalizado pero no se me ocurre cómo.

Gracias de antemano