彼の手紙に返信を出すべきだったのに。

tbd

You should have replied to his letter.

折り返しの見積書をメールで返信してください。

tbd

We look forward to receiving your quote by return mail.

参加できない場合には、返信をください。

tbd

If you cannot join us, please reply.

ようやく、この三週間に受信したメールの返信を書く時間ができました。

tbd

Finally I have time to reply to the mail I received these three weeks.

Finally, I have time to reply to the mail that I have received these past three weeks.

I finally have time to reply to the mail that I have received these past three weeks.

すぐに返信をください。

tbd

Please write me a reply soon.

これを受け取ったらすぐに返信してください。

tbd

Please send me a reply as soon as you receive this mail.