Calling payment is easy but to make it work isn't not so easy:
fluwx.pay(
which: Payment(
appId: result['appid'].toString(),
partnerId: result['partnerid'].toString(),
prepayId: result['prepayid'].toString(),
packageValue: result['package'].toString(),
nonceStr: result['noncestr'].toString(),
timestamp: result['timestamp'],
sign: result['sign'].toString(),
));
Take a look at payment document for help.