- Business type: Two-person brand strategy consultancy, Germany, 15–20 active clients
- Problem: Overdue invoices being followed up inconsistently due to time pressure and discomfort with payment conversations
- Agent type: Invoice monitoring + escalating outreach agent
- Platform used: Zapier + Lexoffice API + Gmail
- Monthly running cost: €31/month
- Status: Running since March 2025
The real problem: it's not about money, it's about the conversation
Late payments are a structural problem for small consultancies. Clients aren't always late because they're trying to avoid paying — often they're just busy, and an unpaid invoice isn't in their active attention. The consultancy following up is, for many freelancers and small firms, genuinely uncomfortable. It feels like asking for something you shouldn't need to ask for.
The two partners here, Lena and Marc, had a consistent pattern: invoices went out, some got paid promptly, and somewhere between 15% and 20% quietly slipped past their due date. Then nothing happened until one of them noticed it on a weekend and felt guilty about not having followed up sooner, which made the follow-up email feel more fraught than it needed to be.
They weren't losing clients over this. But they were consistently carrying between €3,000 and €6,000 in outstanding receivables at any given time, which occasionally created cash flow awkwardness.
How the agent works
The agent monitors Lexoffice (their accounting software) daily via API. When an invoice passes its due date without a recorded payment, it triggers an escalating sequence:
- Day 1 overdue: A short, neutral reminder — "Just a note that invoice [number] for [project] was due on [date]. Happy to help if there are any questions." No pressure language.
- Day 8 overdue: A slightly more direct message — "Following up on the outstanding invoice. Please let us know if you need any information to process payment."
- Day 18 overdue: A message that flags it as requiring attention and offers a call to discuss. This one goes to both Lena and Marc for review before sending — they didn't want the agent sending this autonomously.
All messages are drafted using an OpenAI API call with a custom system prompt that Lena wrote. The prompt includes instructions about tone, their firm's name, and specific language they want to avoid ("regrettably," "as per our records," anything that sounds like a debt collection template).
The Day 18 message review step was a deliberate choice, not a technical limitation. They wanted to stay involved in cases that had reached that point, because those situations sometimes involve a client who is genuinely struggling, not just disorganized.
Results in 60 days
Over the first 60 days, the agent sent 23 first-reminders, 11 second-reminders, and flagged 4 invoices for the Day 18 review. Of the 23 invoices that triggered the first reminder, 19 were paid within 5 days of the message. Three required the second reminder. One escalated to the review step and was eventually resolved via a call — a client who'd had an internal approval process delay.
The €4,200 figure is the total value of invoices paid within 7 days of an automated reminder that had been overdue for more than 3 days at the time the agent flagged them. Lena's estimate is that roughly half of that would have been paid eventually anyway, but later — the agent accelerated collection rather than creating it from nothing.
The part they didn't expect: the messages feel less awkward for clients to receive too. Several clients mentioned it unprompted, saying the reminder was "helpful" or that they appreciated the neutral tone. The consultancy relationship didn't suffer.
The agent can't tell the difference between a payment that's been received but not yet marked in Lexoffice and a payment that genuinely hasn't arrived. In the first two weeks, it sent two reminders for invoices that had already been paid but where the bank reconciliation was a few days behind. Lena now marks invoices as paid in Lexoffice on the same day payment arrives, not when the reconciliation runs. It's a small process change that took about a week to become habit.
If this is relevant to your situation
The Lexoffice integration is specific to German-market accounting software, but the same pattern applies to any invoicing system with an API — FreeAgent, HoneyBook, Wave, FreshBooks, and QuickBooks all have documented APIs that support this kind of automation. The Zapier connection is the simplest entry point if you're not comfortable with direct API calls.
The most important decision you'll make is the tone of the messages. Spend time on the system prompt or the templates. A late payment reminder that sounds like a debt collection notice will damage client relationships. The goal is to make paying easy, not to make not paying feel punished.