A brief email to the client should suffice; there is no need for a formal letter.

給客戶一封簡短的電子郵件就夠了，不需要寫正式信函。

Two hours will suffice for the budget meeting if everyone stays on topic.

如果大家都不離題，預算會議兩小時就足夠了。

Standard shipping usually suffices for orders within Taiwan, so express delivery is optional.

台灣境內的訂單通常用標準運送就足夠，快遞可自行選擇。