400 Bad Request: INVALID_REQUEST, 필수 파라미터 누락

        RestTemplate rest = new RestTemplate();

        HttpHeaders headers = new HttpHeaders();

        String testSecretApiKey = tossPaymentConfig.getTestSecretKey() + ":";
        String encodedAuth = new String(Base64.getEncoder().encode(testSecretApiKey.getBytes(StandardCharsets.UTF_8)));

        headers.setBasicAuth(encodedAuth);
        headers.setContentType(MediaType.APPLICATION_JSON);
        headers.setAccept(Collections.singletonList(MediaType.APPLICATION_JSON));

        JSONObject param = new JSONObject();
        param.put("orderId", orderId);
        param.put("amount", amount);
        param.put("paymentKey", paymentKey);

        //System.out.println(new HttpEntity<>(param, headers));
        return rest.postForEntity(
                PAYMENT_URL + paymentKey,
                new HttpEntity<>(param, headers),
                TossPaymentSuccessDto.class
        ).getBody();