お支払日が先月末の請求書５６７８９について、まだお支払いを受けておりません。
With reference to the invoice No.56789 which was due last month, we have not yet received your remittance.

もしも、すでにお支払い済みでしたら、どうぞこのメールは無視してください。
Should you have settled the account since this mail was written, please ignore our reminder.

支払能力は全く気質の問題であって、収入の問題ではない。
Solvency is entirely a matter of temperament and not of income.