You can send the letter by airmail ( regular mail, registered mail ).

你可以邮寄航空信 ( 平信 、 挂号信 ).

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The postage airmail is greater than that for regular mail.

航空信的邮资较平信为高.

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The postage for airmail is greater than that for regular mail.

航空信的邮资比平信高.

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Even though it is now called snail mail, regular mail is anything but slow.

虽然它现在被叫做蜗牛信, 但普通信件一点也不慢.

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