Price the extras when a client keeps adding work

Help me deal with a client who keeps adding extra work.

The original job and price: [details]
Extras they've asked for: [list]
Already done for free: [list, if any]
How I'd charge for extras: [e.g. hourly rate, fixed price per item]
How I usually talk to this client: [e.g. friendly, formal]

Ask me up to 3 short questions first. Wait for my answers.

Then:
1. A friendly message that says yes to the extras, with a price for each, and asks them to confirm before I start.
2. A short variation note: original job, extra work, price, effect on timescale, and a line to approve.
3. What to say in person next time they say "while you're here…".
4. A sentence to add to future quotes so this is clear from the start.

Friendly, not petty, UK English.

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