。
Don't forget to send the letter.
Literal
Not-forget-[without-ず] letter [object-を] submit-[soft-command-なさい]
A slightly unusual verb choice: 投稿 normally means 'to submit (for publication)' — as in submitting a manuscript, reader letter, or online post. Using it for mailing a private letter is nonstandard; 投函 or 出す would be the idiomatic choice here. The sentence reads as either a mistake or as 'submit the letter (for publication)'.