。
Don't forget to put that letter in the mailbox.
Literal
Not-forget-[without-ず] that letter [object-を] post-box [into-に] put-[soft-command-なさい]
ポスト here is the loanword from English 'post' but narrowed to mean 'mailbox/postbox' — the red cylindrical street boxes for outgoing mail. に marks where the letter goes into. This is the most literal, kid-friendly alternative to 投函.