SubjectInvoice [Invoice #] due [Due date]
+ 2 more subject options
Hi [Client first name],
Just a quick heads-up that invoice [Invoice #] for [Amount] is due on [Due date]. I've attached it here so you have it handy.
You can pay here: [Payment link]
If anything needs to change on your end, like a PO number or a different contact in accounts payable, let me know and I'll take care of it.
Thanks,
[Your name]