public function post() {
        $opts = $this->module->getActingOpts();
        $payload = [
            'webCheckoutDetails' => [
                'checkoutReviewReturnUrl' => $this->checkoutReviewUrl,
            ],
            'platformId' => Constants::PLATFORM_ID,
            'storeId' => $opts['client_id'],
        ];
        $headers = ['x-amz-pay-Idempotency-Key' => uniqid()];

        try {
            $client = $this->module->getAmazonClient();
            // call `generateButtonSignature` so that an error is thrown if the merchant's
            // public/private key pair is not correct
            $client->generateButtonSignature($payload);
            $result = $client->createCheckoutSession($payload, $headers);
            if ($this->module->errors->hasError($result)) {
                return $this->module->errors->getAmzErrorResponse($result);
            }
        } catch (\Exception $e) {
            return $this->module->errors->getErrorResponse(
                GenericErrorStore::AMAZON_PAY_SDK_CLIENT_EXCEPTION,
                [$e->getMessage()]
            );
        }

        return json_decode($result['response'], true)['checkoutSessionId'];
    }