public static function create($shipment_source_obj)
 {
     switch ($shipment_source_obj->shipping_service) {
         case '':
             $tracking_obj = null;
             break;
         case 'wf_usps':
             WfTrackingFactory::wf_include_once('WfTrackingUSPS', 'class-wf-tracking-usps.php');
             $tracking_obj = new WfTrackingUSPS();
             break;
         case 'wf_canada_post':
             WfTrackingFactory::wf_include_once('WfTrackingCanadaPost', 'class-wf-tracking-canadapost.php');
             $tracking_obj = new WfTrackingCanadaPost();
             break;
         default:
             WfTrackingFactory::wf_include_once('WfTrackingDefault', 'class-wf-tracking-default.php');
             $tracking_obj = new WfTrackingDefault();
             break;
     }
     if (null != $tracking_obj) {
         $tracking_obj->init($shipment_source_obj);
     }
     return $tracking_obj;
 }