public function handleCheckoutResult() {
        global $usces;

        $sessionId = $_REQUEST['amazonCheckoutSessionId'];
        $result = (new API\CheckoutSession\Complete($this->module))->post($sessionId);
        if ($result instanceof GenericError) {
            $this->error = $result;
            $this->setToErrorPage();
            return;
        }

        $this->checkoutResponse = json_decode($result['response'], true);
        $details = $this->checkoutResponse['statusDetails'];
        $checkoutSessionState = new Models\CheckoutSession\State(
            $details['state'],
            $details['reasonCode'],
            $details['reasonDescription']
        );

        if ($checkoutSessionState->isCanceled()) {
            $this->error = $checkoutSessionState->getErrorResponseFromCanceledState();
            $this->setToErrorPage();
            return;
        }

        // deferred authorization
        if ($checkoutSessionState->isOpen()) {
            $res = $usces->order_processing();
            if (strtolower($res) === 'ordercompletion') {
                $this->setToCompletionPage();
            } else {
                $this->setToErrorPage();
            }
            return;
        }

        if ($checkoutSessionState->isCompleted()) {
            $getChargeRes = (new API\Charge\Get($this->module))->get($this->checkoutResponse['chargeId']);
            if ($getChargeRes instanceof GenericError) {
                $this->error = $getChargeRes;
                $this->setToErrorPage();
                return;
            }
            $this->chargeResponse = json_decode($getChargeRes['response'], true);

            $res = $usces->order_processing();
            if (strtolower($res) === 'ordercompletion') {
                $this->setToCompletionPage();
            } else {
                $this->setToErrorPage();
            }
            return;
        }

        $this->error = $this->module->errors->getErrorResponse(Errors\GenericErrorStore::UNKNOWN_ERROR);
        $this->setToErrorPage();
    }