LINE Bot - Return Flex Message

const reply_track = (req, payload) => {
    return request({
        method: `POST`,
        uri: `${LINE_MESSAGING_API}/reply`,
        headers: LINE_HEADER,
        body: JSON.stringify({
            replyToken: req.body.events[0].replyToken,
            messages: [
                {
                    "type": "flex",
                    "altText": "สถานะการส่งของ",
                    "contents": {
                        "type": "carousel",
                        "contents": [payload]
                    } 
                }
            ]
        })
    });
};