Dear Sir or Madam,
亲爱的先生或女士，
I am writing in reply to your letter of 4 September regarding your outstanding invoice.
我写此信是为了回复您 9 月 4 日关于未付发票的信函。
I would be grateful if you could attend to this matter as soon as possible.
如果您能尽快处理此事，我将不胜感激。
If you would like any further information, please don't hesitate to contact me.
如果您想了解更多信息，请与我联系。
I look forward to hearing from you.
我期待收到您的来信。
