private function _authorizePaymentAmount ($authorizationAmount, $authorizationReferenceId)
    {
        $response = $this->callStepAndCheckForException('authorizePaymentAmount', 
                array(
                    $authorizationAmount,
                    $authorizationReferenceId
                ));
        printAuthorizeOnBillingAgreementResponse($response);
        return $response->getAuthorizeOnBillingAgreementResult()
            ->getAuthorizationDetails()
            ->getAmazonAuthorizationId();
    }