。
Please don't forget to mail this letter.
Literal
Not-forget-[without-ず] this letter [object-を] send-[polite-request-てください]
Colloquial alternative — 出す instead of 投函. Japanese has different verbs for the same real-world action (出す, 投函, 送る, 投稿) that span everyday speech, formal mailing language, and publication-submission senses; learners benefit from seeing them swapped in similar frames.