public function getAmazonOrderReferenceId()
    {
        $paymentHandler = $this->GetPaymentHandler();
        if (false === ($paymentHandler instanceof AmazonPaymentHandler)) {
            throw new \InvalidArgumentException('order was not paid with amazon payment');
        }

        /** @var $paymentHandler AmazonPaymentHandler */

        return $paymentHandler->getAmazonOrderReferenceId();
    }