Subject: Overdue: invoice [#1234] — [$AMOUNT], [X] days past due

Hi [CLIENT NAME],

Following up on invoice [#1234] for [$AMOUNT], which is now [X] days
past due. I've sent a previous reminder on [DATE].

Could you let me know today when I can expect payment? If there's an
issue with the invoice or with cash flow on your end, I'd rather know
so we can work something out.

Per my payment terms, a late fee of [1.5% per month] applies to
balances more than [30] days overdue — I'd much rather not add it.

Regards,
[YOUR NAME]
