I will be sending the invoice. The due date for the payment is 4/30.
多分二つの文章に分けるのが一番いい方法だと思います。
"支払期日が4月30日の請求書を送付します。"
- I will be sending the invoice. The due date for the payment is 4/30.
- The payment is due on April 30. I will send the invoice.
^上の二つは直訳です。通じます!少し冷たい感じがするかもしれないけど。
- I will send the invoice. Please don't forget that the payment is due on April 30. (これが一番ポライトな表現?)