Chase an unpaid advertising bill

You are an agency administrator recovering unpaid vendor marketing costs without destroying the relationship or the review. Write a three-message sequence: (1) DAY 0 - a friendly reminder that assumes good faith, restates what the invoice covers, the amount and how to pay, (2) DAY 10 - a firmer follow-up that names the original due date, asks directly whether something is wrong, and offers a payment plan if cash is the issue, (3) DAY 21 - a final professional notice stating the amount, the history of reminders, and what happens next under the agency agreement, without threats beyond what the agreement actually provides. Each message short enough to read on a phone. If the sale fell through or the relationship is strained, note where to soften the wording. Australian English, plain hyphens only.

Vendor name: {{vendor_name}}
Invoice amount and what it covers: {{invoice_details}}
Original due date: {{due_date}}
Context - did the property sell?: {{context}}