public function sessionIdExistsOrExit() {
        return function () {
            if (!isset($_REQUEST['amazonCheckoutSessionId'])) {
                header('location: ' . get_option('home'));
                exit;
            } else {
                $sessionId = $_REQUEST['amazonCheckoutSessionId'];
                $result = (new CheckoutSession\Get($this->module))->get($sessionId);
                if ($result instanceof GenericError) {
                    header('location: ' . get_option('home'));
                    exit;
                }
            }
        };
    }