the OP as an American is not eligible for VOA and should use the following site to apply for a 60-day tourist visa, or he can arrive in Thailand visa-exempt for a 30 day stay (extendable for another 30 days).
I don't know if this will work but the US government has something called pay.gov:
"Pay.gov lets individuals, states and businesses make non-tax related payments to the federal government using the internet. Payments can be made to federal agencies using a U.S.-held bank account (through ACH Debit), a credit or debit card, or with a digital wallet, such as PayPal or Amazon Pay."
First, look up the US government department you wish to pay and see if they participate in gov.pay:
One of the ways of extending a non-OA is by having 800k baht in a thai bank 2 months before you apply. Has nothing to do with having a pension income and needing a letter from your enbassy.
The other way is by getting your embassy to issue a letter that you have income of at least 65k per month.
So it's either one or the other. Lump sum of 800k in bank and no embassy needed or income of 65k/month and embsssy letter needed.
Why do you need an embassy letter if you are doing the 800k lump sum method, which is not the income method?