I decided to make her a card instead, and then I could stick it in her mailbox, or leave it off at her room.

我想到做一个卡片，然后塞进她的信箱或者悄悄放进她的房间。

youdao

The mail room personnel create an envelope by using the recipient information in your letter, seal your letter in an envelope, and drop the envelope in the mailbox for delivery.

收发室人员使用信件中的收件人信息制作信封，并将信件封在信封中，然后将信封投入信箱以进行投递。

youdao

There was a time when it was hard to enter a NYC taxicab or men's room, or to open a mailbox to mail a letter, without confronting a sticker hawking Henry's latest effort.

WSJ: Hollywood Moments