/**
* Don't add item to the collection if only fixed are allowed and its carrier is not fixed
*
* @param \Magento\Quote\Model\Quote\Address\Rate $rate
* @return $this
*/
public function addItem(\Magento\Framework\Object $rate)
{
$carrier = $this->_carrierFactory->get($rate->getCarrier());
if ($this->_allowFixedOnly && (!$carrier || !$carrier->isFixed())) {
return $this;
}
return parent::addItem($rate);
}