Hi {{first_name}},
This is a final notice regarding Invoice [[invoice number]] for [[amount]], now [[number]] days overdue. I've sent several reminders without a response.
If payment is not received by [[date]], I will need to [[specific consequence per your contract: apply the late fee outlined in our agreement, refer this account to collections, or engage a third party for resolution]].
I'd much rather resolve this directly. Please reply with a payment date or call me at [[your phone number]] to discuss.
[[Your name]]