@David *****
yes. Sadly corruption is strong. I coverted from a visa exempt to a 3 month Non O when I returned after COVID around 2021.
One I arrived visa exempt, I went to Chaeng Wattana with all documentation including 12 months of international transfers of THB 65,000. To my surprise, Chaeng Wattana would not accept the monthly transfers; they would only accept THB 800,000 in a Thai bank or a letter from my embassy if they still issued (U.S. does not.).
I had to scramble and sell stocks and transfer the money to THB 800,000. On all my yearly extensions since then, I use the THB 65,000 and have not had a problem and I have not needed an agent. But I can certainly understand why people use agents.