1. Letters will cost a little more, but they will certainly travel faster.
    这样会稍微多花点钱，但肯定是快得多了。

2. I wrote several letters on his behalf.
    我代表他写了好几封信。

3. I need another file for my letters.
    我还需要一个存放信件的夹子.

4. In the course of his lifetime he wrote thousands and thousands of letters.
    在他的一生中，他写了上千封信。