會發生此原因可能是google play的後臺彈窗顯示權限沒開,
用小米手機的話必須將google play的"後臺彈窗顯示"設為允許,
這樣就能使用內購服務了.
This In-APP billing error may happens on Xiaomi serials phone.
You have to go to google play APP setting and change permission to allow of "enable popup windows in background".
