The short version: a final notice works because it's concrete (a real date, a real consequence) and because it's genuinely final. Send it around day 30, after earlier reminders were ignored, and be prepared to follow through. Two proven variants below.
If you haven't sent the earlier stages yet, start with the full reminder ladder. A final notice out of nowhere burns goodwill you may still want.
Variant 1: The professional ultimatum
Subject: Final notice: invoice [#1234] ([$1,500])
[Client name],
Despite reminders on [July 7] and [July 21], invoice [#1234] for [$1,500] (due [June 30]) remains unpaid.
If payment is not received by [August 5], I will:
pause all current and scheduled work, and
refer the balance to a collections service.
I'd much rather resolve this directly. You can settle it in one click here: [payment link], or reply today if there's a genuine issue I should know about.
[Your name]
Why it works: dated reminder history (you kept receipts), a specific deadline, consequences listed as facts rather than threats, and one last open door.
Variant 2: The third-party handoff
Subject: Invoice [#1234]: escalation notice
Hi [Client name],
As of this week, my accountant handles all invoices more than 30 days overdue, and invoice [#1234] ([$1,500], due [June 30]) is now in that queue.
If it's paid by [August 5], I'll close the file and we're square: [payment link]
After that date it's out of my hands, and their process involves formal demand letters and collections referral.
[Your name]
Why it works: the third-party framing removes the personal standoff: the client is no longer negotiating with you, and you're no longer the bad guy. Freelancers consistently report this version producing the fastest payments of any email in the sequence. (It's fine if "my accountant" is a process rather than a person, but the escalation itself must be real.)
If the final notice is ignored
Follow through on the stated date, without another warning. Options and the cold math for each (collections agencies take 25-50%, small claims is worth it above certain amounts, and some invoices deserve writing off along with the client) are covered in what to do when a client doesn't pay.
The best final notice, though, is the one you never send because days 0-21 were handled consistently. DunPilot runs that ladder for you with tone-matched drafts, promise tracking, and an escalation path that never forgets, so invoices rarely age far enough to need an ultimatum.
Tired of chasing invoices by hand?
DunPilot finds your overdue invoices and drafts reminders tuned to each client's payment history. Nothing sends until you approve.