Ok, your embassy just give affidavit letter based on your income. You’ll need to provide your own bank statement in Thailand directly to the immigration yourself.
If you’re in the process of applying for a 90 days Non O visa at home, you’ll just use your bank at home, but for the extension of stay in Thailand you’ll need to transfer your funds to a bank in Thailand.
You are from the US and as John explains you’ll need to show proof of funds in your bank in Thailand to apply for an extension of stay at your current visa.
Did you apply for a 90 days Non O visa at home before you came? What date did you enter Thailand? Do you have a bank account in Thailand?
Now I’m in Norway 🇳🇴 and they still charge me 15 thb in Thailand 🇹🇭. 😉They also charge me 15 thb when I withdraw money out of province (called transfer fee), but not when I use the scan function in the app to pay.