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The late-invoice chaser: escalating payment reminders without a €500/month SaaS

Polite, automatic follow-ups that get you paid on time

Automatic, polite, escalating reminders on every overdue invoice, plus a Monday-morning cash-flow digest — self-hosted, and the awkward final email stays human.

The problem

You did the work, sent the invoice, and now it sits. Chasing feels awkward, so it happens in bursts — usually when cash gets tight, which is the worst possible moment to discover that three clients are 40+ days late. The dedicated AR platforms solve this for $180–900 per month, priced for companies with a finance team. Meanwhile most small-business "automation" content on this topic is a listicle of email templates you still have to remember to send. The actual fix is boring: a system that never forgets, escalates on a fixed schedule, and puts one short exception report in front of you weekly.

The architecture

Two separate loops, on purpose. Routine reminders (D-3 to D+14) are template-driven and fully automatic — no AI, nothing to hallucinate, they just fire. The 30+ day escalation is where relationships and judgment live, so the machine only drafts and a human decides. Most AR SaaS sells you the first loop at platform prices; you can self-host it.

Tool choices — and why

Invoice Ninja, self-hosted (9.9k stars, active — last push 30/06/2026). A full invoicing system with the reminder engine built in: three configurable reminders plus recurring "endless" reminders after that, each with its own template, timing relative to the due date, and optional late fee. It also gives you a client portal and online payment links — the single biggest lever for getting paid faster, because "click here and pay" beats "please wire to IBAN..." every time. License note: Elastic License 2.0, not OSI open source — self-hosting it to run your own business is explicitly fine; offering it to others as a hosted service is not. The free self-hosted edition shows small Invoice Ninja branding on client-facing documents; a paid white-label option exists if that bothers you. Alternatives: if you already invoice from accounting software that has reminders (e.g. your bookkeeper's platform), use those and keep only the digest loop below — don't run two invoicing systems.

n8n (194.8k stars, active — last push 01/07/2026). One scheduled workflow: call the Invoice Ninja API, bucket open invoices by age, format a digest, send it to you. This is deliberately read-only against the invoice system — the workflow can't modify invoices, so a bug produces a wrong report, not a wrong invoice. Fair-code Sustainable Use License; self-hosting for your own business is allowed.

Ollama + a small instruct model (MIT, ~175k stars, active). Used for exactly one thing: turning "Client X, 3 invoices, 34 days, €4,200, second escalation" into a short, firm, personal email in the client's language. Payment history stays on your machine. An 8B-class model does this well because the input is structured and the output is four sentences. If you skip the LLM entirely, the blueprint still works — you just write the escalation emails yourself from the digest.

Monthly cost. Software: €0. Hardware: a €5–12/month VPS runs Invoice Ninja + n8n comfortably; add the LLM only if the box has ~8 GB+ RAM, otherwise run Ollama on an office machine. Payment-link fees are the real cost: card/PSP processing typically runs ~1.5–3% per transaction depending on provider and country (verify current rates for your PSP) — for B2B, offering bank transfer with a structured reference alongside cards keeps this near zero.

Setup outline

1. Self-host Invoice Ninja (official Docker image) and migrate active clients + open invoices. Connect your SMTP so reminders come from your own domain.
2. Configure the three reminders: D-3 friendly, D+7 overdue, D+14 firm — each with a payment link. Write them once, in your voice, per language you invoice in.
3. Decide the late-fee question consciously. EU B2B rules (Directive 2011/7/EU, as implemented in your country) allow statutory interest plus a fixed recovery cost on late business payments — whether enforcing that is worth the relationship damage is a business call, not a technical one. This is general information, not legal advice.
4. Build the n8n Monday digest: Invoice Ninja API → aging buckets → one message. Keep it under 15 lines; if you won't read it, nothing else matters.
5. Add the escalation drafter: for 30+ day invoices, LLM drafts from structured facts only (client, amounts, dates, prior reminders) → saved as draft or sent to you for approval.
6. Run one full month, then check two numbers: average days-to-paid before vs after, and how many escalation drafts you actually sent unedited. Tune the templates, not the architecture.

Pitfalls — the real ones

Never auto-send the escalation. Reminder 1–3 are expected business hygiene; nobody resents them. The 30+ day email lands differently — one wrong tone to your biggest client costs more than a year of the automation saves. Draft-only is the design.
Reminders only work if invoice data is clean. Wrong due dates, invoices marked sent that never were, part-payments not recorded — the automation amplifies whatever is in the system. Fix intake first (our receipt-intake blueprint is the sibling of this one).
Don't chase disputed invoices. Add a "disputed" flag workflow: one label in Invoice Ninja pauses reminders for that invoice. An automated dunning email on a disputed amount is how you turn a billing question into a lost client.
Escalation is a process, not an email. After the drafted 30+ day email, the next steps are a phone call and, in serious cases, formal action — humans, not workflows. The blueprint's job ends at making sure you know, every Monday, exactly who to call.
GDPR footnote: payment behavior is personal data when your clients are sole traders. Self-hosting keeps it on your infrastructure; if you swap in a hosted LLM API for drafting, you're shipping client payment history to a third party — don't, the local model is more than enough here.

Verified repos

Invoice Ninja — Elastic License 2.0, 9.9k stars, active (June 2026)
n8n — Sustainable Use License (fair-code), 194.8k stars, active (July 2026)
Ollama — MIT, ~175k stars, active (June 2026)