我正在尝试使用 Google Apps Script 和 Stripe 制作一个插件，用户可以按年订阅某个商品。每次我从 Stripe 结账处购买订阅时，都会收到这样的错误，

{
  "error": {
   "code": "parameter_unknown",
   "doc_url": "https://stripe.com/docs/error-codes/parameter-unknown",
   "message": "Received unknown parameter: @45b5a607",
   "param": "@45b5a607",
   "type": "invalid_request_error"
  }
}

当我在 Stripe 仪表板中检查日志时，我得到了这样的 POST 正文，

{
  "items": "[Ljava.lang.Object",
  "@45b5a607": null,
  "customer": "cus_Dix0eSYM5qP0kx"
}

这是我在 Google Apps Script 中的代码，

var headers = {
    "Authorization" : "Basic " + Utilities.base64Encode(USERNAME + ':' + PASSWORD)
};

var customer = {
  'email': customerEmail,
  'source': token
};

var optCreate = {
  'method' : 'post',
  "headers" : headers,
  'contentType': 'application/x-www-form-urlencoded',
  'payload' : customer,
  'muteHttpExceptions' : true
};

var createCustomer = UrlFetchApp.fetch(urlCreate, optCreate);
var respCreate = JSON.parse(createCustomer.getContentText());
var customerId = respCreate.id;
if (customerId == null) { return "Error"; }

var data = {
  "customer" : customerId,
  "items" : [
    {
      "plan" : "plan_Diuw7CdAGcSrhm"
    }
  ]
};

var options = {
  'method' : 'post',
  "headers" : headers,
  'contentType': 'application/x-www-form-urlencoded',
  'payload' : data,
  'muteHttpExceptions' : true
};

var response = UrlFetchApp.fetch(url, options);
var resp = JSON.parse(response.getContentText());
Logger.log(resp);

我想我的 data JSON 对象中一定做错了什么。 items 字段无法正常工作，这就是 POST 正文奇怪的原因。这里正确的方法是什么？