That is annoying. In Chonburi - Pattaya, no need of certificate. Is also ridiculous, as long you ve the transfers every month. I did provide them with a contract, but they never ask for it.
Should be on your statement. Normal international funds transfer is enough, but should be there. And verify this every month. I did same as you want to do.
I had the 800.000 method first, then changed to monthly income. Jomtien immigration was fine with years statement, where monthly transfer was visible. No affidavit needed, I gave proof, where it came from. But not one transfer in that year can be skipped, or done after the month, it is supposed to be in your account. Not used agency.
Thank you, I kept the 800.000 until I renew this month and change to monthly transfer method. I went to immigration, when applied for re entry permit 2 months ago, and they said, that bankstatement is fine and not need from embassy. In my case impossible, my income is payback from a loan, and it not taxable.
I went to immigration in Jomtien. They accept a proof of monthly transfer, of minimum 65.000 bht. If stated by bank, over the last 12.000 bht. Then no month can be missing though. I asked, because I am tired to hold 800.000 in bank.