Here's my form letter:

Dear _,

I am writing regarding [describe problem].

The appropriate resolution to this issue is [describe solution].

[Usually the solution is cash; if it is cash:] You may credit this to the form of payment you have on file, or by sending me a check at [address].

If you have any questions, you may call me at [phone number] at [time when I'm always free on a weekday during working hours in all us timezones].

I will contact my AG's office if I do not hear back from you in 30 days.

Thanks, [Name]

Mail it.

All state AG offices have a form for consumer complaints on their website. Usually it's very easy to find. Don't overthink it. Mention the letter in your AG complaint, include a copy and the date that you sent it. They will contact the business and start a resolution process. Usually you'll hear back in a week or so.

AFAIK, every state's attorney general's office will have an online site and/or form for filing complaint(s), as well as a postal address you can use.

You can free-form your own letter, though that might delay things somewhat from the AG's office, though the business / countereparty will typically get on their game quickly.

DDG "consumer complaint" + <YOUR_STATE_HERE> should work.

E.g., top ten states by population, covering over half the US population, first result is the corresponding state's AG consumer affairs office:

CA: <https://duckduckgo.com/?q=consumert+complain+california> gives <https://www.dca.ca.gov/consumers/complaints/index.shtml>

TX: <https://duckduckgo.com/?q=consumer+complaint+texas> <https://www.texasattorneygeneral.gov/consumer-protection>

FL: <https://duckduckgo.com/?q=consumer+complaint+florida> <https://www.stateofflorida.com/consumer-complaints/>

NY: <https://duckduckgo.com/?q=consumer+complaint+new+york> <https://dos.ny.gov/file-consumer-complaint>

PA: <https://duckduckgo.com/?q=consumer+complaint+pennsylvania> <https://www.attorneygeneral.gov/submit-a-complaint/consumer-...>

IL: <https://duckduckgo.com/?q=consumer+complaint+illinois> <http://www.illinoisattorneygeneral.gov/consumers/filecomplai...>

OH: <https://duckduckgo.com/?q=consumer+complaint+ohio> <https://www.ohioattorneygeneral.gov/Individuals-and-Families...>

GA: <https://duckduckgo.com/?q=consumer+complaint+georgia> <https://consumer.georgia.gov/>

NC: <https://duckduckgo.com/?q=consumer+complaint+north+carolina> <https://ncdoj.gov/file-a-complaint/consumer-complaint/>

MI: <https://duckduckgo.com/?q=consumer+complaint+michigan> <https://www.michigan.gov/ag/complaints>