/**
  * Retrieve shipping carriers for specified order
  *
  * @param Mage_Eav_Model_Entity_Abstract $object
  * @return array
  */
 protected function _getCarriers($object)
 {
     $carriers = array();
     $carrierInstances = Mage::getSingleton('shipping/config')->getAllCarriers($object->getStoreId());
     $carriers['custom'] = Mage::helper('sales')->__('Custom Value');
     foreach ($carrierInstances as $code => $carrier) {
         if ($carrier->isTrackingAvailable()) {
             $carriers[$code] = $carrier->getConfigData('title');
         }
     }
     return $carriers;
 }