1. Four twenty-five cent stamps, please.
请给我4张25分的邮票。
还能这样说：
I want to buy four twenty-five cent stamps, please.
I'd like four twenty-five cent stamps, please.
应用解析：
bear the stamp of 带……标记;具有……迹象;
stamp down 镇压，践踏;
stamp sth. on 在……上盖印，留下印记;惩治
2. Can you help me mail this letter?
你能帮我寄这封信吗?
还能这样说：
Would you do me a favor to mail the letter?
Could you be kind to post the letter for me?
应用解析：
mailing list 邮寄名单;
mail order 邮购制度;
mails hot (寄给潜在客户的)广告材料;
the mailed fist 威胁，武力威胁
3. I want to post this letter.
我想寄这封信。
还能这样说：
I have a letter to post.
I have a letter to mail.
应用解析：
keep sb. posted 使某人不断获悉最新发展情况或消息等;
post-haste 火速的，赶紧;
post process 后期处理
4. I want this letter registered.
我想寄挂号信。
还能这样说：
By registered letter, please.
I prefer to have the letter registered.
应用解析：
by letter 以书信形式;
call letters (电台)呼号;
in letter and in spirit 无论形式和实际;
claim letter 索赔书