public function authorizePaymentAmount ($authorizationAmount, $authorizationReferenceId)
    {
        $authorizeOnBillingAgreementRequest = $this->_createAuthorizeOnBillingAgreementRequest(
                $authorizationAmount, $authorizationReferenceId, false);
        return $this->_service->authorizeOnBillingAgreement($authorizeOnBillingAgreementRequest);
    }