That can easily be done via the official website that is mentioned by Graham Seal. But don't use your telephone for it, but a laptop or pc, the screen of a telephone is just not big enough. When the price is still the same, it's € 90.
You can better save your money for an agent in Thailand if you need one, but you could also easily do this yourself. The Dutch embassy still provides an affidavit about your income, see
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With this letter you don't need to open een bank account to put 800k Thai baht on it. Just arrange a TM30 with your landlord (if you hire a house or an appartement) or with the owner of the house you stay in, friend or partner. In that case, he or she can come with you when you arrive and fill in a TM30 at the Immigration Office. For the extension of your visa, possible after two months after your arrival, your friend or partner can best come with you to the Immigration Office again. Pay 1.900 baht and basically that's it.