Write a complaint letter that gets a refund

I need to write a complaint to a company and I want it to get results.

What I bought and what went wrong: Bought a washing machine from an appliance store on July 12, order 58-22910, $649 including delivery. It started leaking from the bottom on August 30. Their repair visit on September 9 couldn't fix it and the technician said the drum seal was faulty.
What I want: a full refund
What I've already tried: Called twice. Second time a manager called Luis said he'd 'look into a replacement' and would call back within 48 hours. That was 11 days ago.
Where I live: Texas, US

Write the complaint. Under 250 words.

Structure:
- First paragraph: what I'm complaining about and what I want, in two sentences. They should know both before reading anything else.
- Then the facts in date order, short and specific. Order number, dates, names, what was promised.
- Then what I want again, with a clear deadline for a reply (14 days unless you think a different deadline fits better), and what I'll do if I don't get one.

Rules:
- Firm and polite. No sarcasm, no capital letters, no insults, even if I sound angry above.
- Don't cite a specific law or regulation unless you're confident it applies where I live. If consumer rights where I live are likely to help me, tell me separately which official body or consumer organization I could check with, rather than putting a legal claim in the letter.
- Only use facts I gave you.

Then tell me:
1. Who to send it to and how, if a phone call has already failed.
2. What evidence to keep and attach.
3. What my next step is if the deadline passes, in two lines.

What you get: A firm complaint that leads with what you want and sets a deadline, plus a plan for when they miss it.