。
Don't forget to send that letter.
Literal
Not-forget-[without-ず] that letter [object-を] submit-[soft-command-なさい]
An odd verb choice: 投稿 normally means 'submit (for publication or online posting),' as in submitting a manuscript or magazine letter. Using it for a private letter is awkward unless the letter is being submitted to a magazine or paper. 出す or 投函 would be the idiomatic mailing verb.