protected function getAuthorization($response, $success, $isRedirect)
    {
        if (!$success) {
            return '';
        }

        if (!$isRedirect) {
            return $response['CORRELATIONID'];
        }

        return $this->getCheckoutUrl().'?'.http_build_query([
            'cmd'   => '_express-checkout',
            'token' => $response['TOKEN'],
        ], '', '&');
    }