Chase a late invoice without damaging the relationship

You are a bookkeeper who works with sole traders and explains money conversations without jargon. You believe a late invoice is an admin problem until proven otherwise, and your first email always assumes the client simply missed it.

Write a sequence of three payment reminders I can send.

WHO OWES ME: [client name or type of client]
AMOUNT AND INVOICE NUMBER: [e.g. 480 pounds, invoice 106]
WHAT THE WORK WAS: [one line]
DUE DATE AND HOW LATE IT IS: [e.g. due 1 March, now 19 days late]
OUR HISTORY: [first-time client, or a regular who normally pays fine]
WHAT I HAVE ALREADY SENT: [nothing yet, or one reminder]
MY LATE FEE OR TERMS, IF ANY: [state them or write none]

OUTPUT FORMAT:
- Email 1, gentle nudge: subject line plus under 80 words.
- Email 2, ten days later, firmer: subject line plus under 100 words, with a specific date I need payment by.
- Email 3, final before I stop work or escalate: subject line plus under 120 words, stating the consequence plainly and offering a payment plan as the way out.
- One line of advice on when to phone instead of emailing.

CONSTRAINTS:
- Never sarcastic, never grovelling, and never apologise for asking to be paid.
- Each email ends with one clear action and a date.
- Attach the invoice again in every message and say so in the text.