Five follow-up emails for an invoice past 30 days
https://archon-partners.com/guides/unpaid-invoice-follow-up/

From Archon & Partners. Replace everything in [square brackets], read each one, and use it from your own accounts. Check your agreement before mentioning interest, suspension or any contract term. This is not legal or tax advice.

=== 1. Thirty days past due: a statement and a direct question ===
When: Day 30. Send to your project contact and copy accounts payable. Call the same day.
Subject: Invoice [number] for [project]: 30 days past due

Hi [first name],

Invoice [number] for [project] ([amount]) was due on [due date] and is now 30 days past due. I have attached the invoice and a statement of everything open on the account.

Could you tell me what is holding it up? If something is missing on our side, such as a purchase order number, a supplier form or a revised copy, I will send it today.

I will call you on [day] so we can settle a payment date.

Thank you,
[your name]
[firm name]

=== 2. Escalation to the person who signed the agreement ===
When: Around day 45, if you have no payment date. Keep the project contact copied.
Subject: [Project]: invoice [number] needs your attention

Dear [name of signatory],

I am writing about invoice [number] for [project], for [amount], which was due on [due date]. Despite reminders on [dates] to [project contact name], we do not yet have a payment date.

We want to keep [project] on schedule. Could you confirm by [reply-by date] when this invoice will be paid, or tell me what is preventing payment so we can resolve it?

The invoice and statement are attached. I am available on [your number].

Kind regards,
[your name]
[firm name]

=== 3. Confirming a payment plan ===
When: The same day you agree a plan by phone or in a meeting.
Subject: Payment plan for invoice [number]: confirming what we agreed

Hi [first name],

Thank you for agreeing a plan for invoice [number] ([amount]). To confirm:

- [amount] by [date]
- [amount] by [date]
- [amount] by [date]

If a payment is going to be late, please tell me before the date. If a payment is missed without notice, the full balance becomes due on [date], as we discussed.

Please reply to confirm this is your understanding too.

[your name]
[firm name]

=== 4. Written notice under the agreement ===
When: Day 60 or later. Use the clause numbers and notice method your agreement requires. Have a lawyer read it first.
Subject: Notice under our agreement dated [date]: invoice [number]

Dear [name of signatory],

This is written notice under [section reference] of our agreement dated [agreement date] for [project].

Invoice [number] for [amount], due on [due date], remains unpaid. [If your agreement allows interest: Under [section reference], interest applies to unpaid amounts at [rate in your agreement] from [date].]

[If your agreement allows suspension: Unless payment is received by [date], at least [notice period in your agreement] from today, we will suspend services on [project] under [section reference] until the account is brought up to date.]

Please call me on [your number] before [date].

[your name]
[title], [firm name]

=== 5. Final notice before referral ===
When: When the date in your notice has passed. Name only a next step you will take.
Subject: Final notice: invoice [number] for [project]

Dear [name of signatory],

Invoice [number] for [amount], due on [due date], is still unpaid. We wrote about it on [dates] and gave notice under our agreement on [date].

If payment, or a written payment plan we both sign, is not received by [date], we will refer the matter to [our lawyer / a collection agency] without further notice.

I would still rather resolve this directly. You can reach me on [your number].

[your name]
[title], [firm name]