/**
  * Don't add item to the collection if only fixed are allowed and its carrier is not fixed
  *
  * @param Mage_Sales_Model_Quote_Address_Rate $rate
  * @return Mage_Sales_Model_Resource_Quote_Address_Rate_Collection
  */
 public function addItem(Varien_Object $rate)
 {
     if ($this->_allowFixedOnly && (!$rate->getCarrierInstance() || !$rate->getCarrierInstance()->isFixed())) {
         return $this;
     }
     return parent::addItem($rate);
 }
 /**
  * Overload default addItem method to set environment for the rules
  *
  * @param Mage_Rule_Model_Abstract $rule
  * @return Mage_Rule_Model_Resource_Rule_Collection
  */
 public function addItem(Varien_Object $rule)
 {
     $rule->setEnv($this->getEnv())->setIsCollectionValidated(true);
     parent::addItem($rule);
     return $this;
 }