first-class mail

美 [fɜːrst klæs meɪl]英 [fɜːst klɑːs meɪl]

first-class mail

noun

mail that includes letters and postcards and packages sealed against inspection: Synonym: first class 1st class 1st-class mail

数据来源：WordNet

1

The US Postal Service on Wednesday said it was ending first-class mail deliveries on Saturdays .

美国邮政周三宣布将停止周六的投递服务。
2

Send it by first-class mail .

把它用第一类邮件寄出。
3

Well , we send all the first-class mail by plane to the East Coast , so you don 't need an airmail stamp .

是这样，我们寄到东海岸的一级邮件全部用飞机传递，所以你不需要贴航空邮票。