public static function get_instance()
 {
     if (null === self::$instance) {
         self::$instance = new self();
     }
     return self::$instance;
 }
 public function register_action_links($links, $plugin_file)
 {
     if ($plugin_file === WPEC_Retargeting::get_instance()->get_plugin_name()) {
         $url = admin_url('options-general.php?page=wpsc-settings&tab=retargeting');
         $links[] = '<a href="' . esc_attr($url) . '">' . esc_html__('Settings') . '</a>';
     }
     return $links;
 }