Sort or filter by Shipping method

<?php
/**
 * @package    HikaShop Order Shipping Filter Plugin
 * @version    1.0.1
 * @author     Custom
 * @license    GNU/GPLv3 http://www.gnu.org/licenses/gpl-3.0.html
 */
defined('_JEXEC') or die('Restricted access');

class plgHikashopOrder_shipping_filter extends JPlugin
{
    public function __construct(&$subject, $config) {
        parent::__construct($subject, $config);
        $lang = JFactory::getLanguage();
        $lang->load('plg_hikashop_order_shipping_filter', JPATH_ADMINISTRATOR);
    }

    public function onBeforeOrderListing($paramBase, &$extrafilters, &$pageInfo, &$filters, &$tables, &$searchMap, &$select) {
        $extrafilters['shipping'] = $this;

        $app = JFactory::getApplication();
        $db = JFactory::getDbo();

        $pageInfo->filter->filter_shipping_id = $app->getUserStateFromRequest(
            $paramBase . ".filter_shipping_id",
            'filter_shipping_id',
            '',
            'int'
        );

        if (!empty($pageInfo->filter->filter_shipping_id)) {
            $filters[] = "b.order_shipping_id = " . (int)$pageInfo->filter->filter_shipping_id;
        }
    }

    public function displayFilter($name, $info) {
        $html = '';
        switch($name) {
            case 'shipping':
                $db = JFactory::getDbo();
                $query = $db->getQuery(true)
                    ->select('shipping_id, shipping_name')
                    ->from(hikashop_table('shipping'))
                    ->where('shipping_published = 1')
                    ->order('shipping_name ASC');
                $db->setQuery($query);
                $methods = $db->loadObjectList();

                $html .= '<select name="filter_shipping_id" id="filter_shipping_id" class="custom-select" onchange="document.adminForm.submit();">';
                $html .= '<option value="">' . JText::_('FILTER_BY_SHIPPING_METHOD') . '</option>';
                foreach($methods as $method) {
                    $selected = ($info->filter_shipping_id == $method->shipping_id) ? 'selected="selected"' : '';
                    $label = $method->shipping_id . ' : ' . $method->shipping_name;
                    $html .= '<option value="' . (int)$method->shipping_id . '" ' . $selected . '>' . htmlspecialchars($label, ENT_COMPAT, 'UTF-8') . '</option>';
                }
                $html .= '</select>';

                if(!defined('HIKASHOP_J40') || !HIKASHOP_J40) {
                    $html .= "
                    <style>
                    #filter_shipping_id {
                        width: 250px;
                        display: inline-block;
                        margin-bottom: -9px;
                    }
                    </style>";
                }
                break;
        }
        return $html;
    }
}