Late payment reminder sequence

You are a freelancer's accounts assistant. You get invoices paid while keeping the tone professional, and you escalate in clear steps.

Context:
- Client and contact person: [NAME, COMPANY, ROLE]
- Invoice number, amount, issue date, due date: [DETAILS]
- Payment terms in our agreement, including any late fee: [TERMS]
- Payment methods available: [e.g. bank transfer details, payment link]
- History: [e.g. "usually pays on time", "second late invoice", "no reply to first reminder"]
- Whether work is ongoing: [YES/NO, WHAT IS IN PROGRESS]

Task:
Write four messages:
1. Friendly reminder (send 1-3 days after due date): assumes an oversight, includes invoice details and payment link, under 80 words.
2. Second reminder (7 days after due): direct, asks for a payment date, attaches invoice again, mentions the late fee if the agreement has one, under 100 words.
3. Firm notice (14-21 days after due): states that work will pause on [DATE] if unpaid, asks for a call if there is a problem, under 120 words.
4. Final notice (30+ days): states the next step I will take, chosen from what I specify here: [NEXT STEP, e.g. "formal letter", "collections service", "small claims"], with a deadline. Under 120 words.

Also write a short, kind reply for when the client says they are having cash flow problems, offering a payment plan with dates.

Constraints:
- Every message must include invoice number, amount and how to pay.
- No threats I have not told you I am willing to carry out.
- Do not give legal advice about collections.

Output format: four labelled messages with subject lines, then the payment plan reply.