【求助】关于Fastmail AppStore 订阅

我查到Fastmail 土区的app store订阅价格是大概不到$2，想通过内购的方式来订阅Fastmail，但是我通过iOS 官方app注册的trial账号不能触发应用内订阅，choose a plan地方显示的是空白。客服跟我说我的账号是“It appears that your account is set up for billing directly through Fastmail rather than through an in-app purchase via the App Store.”

求各位大佬指点迷津 ^-^ ^-^