回答

・post in the mailbox
・drop into the mailbox

「投進郵筒」可以用post in the mailbox/drop into the mailbox/mail來表達。

drop into是"投進～到郵筒"
mail有"郵寄，投進～到郵"的意思。

Could you post this in the mailbox for me?
『你能幫我把這個投進郵筒嗎？』

I must drop this application into the mailbox by the end of the day.
『我必須在今天之前將這份申請書郵寄出去』

有幫助｜0