envelope:

n. 信封，封皮包膜[天] 包层包迹

Don't forget to put a stamp on the envelope.

不要忘记在信封上贴邮票。

letter:

n. 信字母，文字证书文学，学问字面意义

vt. 写字母于

vi. 写印刷体字母

Business letters should be written in clear and logical language.

商业信函的语言应当清楚且有逻辑。