Choose your agent carefully. I would advise using one that takes you to immigration so you see the stamp put in and sign all the documents. I used an agent first off and they opened a bank account, put in the money and got my 90 day and 12 month all at the same time. Cost was around
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baht. It does make some things back home difficult with local immigration. They would not put my extension stamp into my new passport so I had to fly to Bangkok. Residence certs are required for a lot of things and they are complicated to get if your stamp is not local.
To transfer to doing it yourself as I did you will need
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0 baht in a thai bank account for 12 months but some offices might be different. Having done that it was not difficult to obtain my stamp myself for 1900 baht.
Looking back I would choose to do it myself from day one if it was possible. For me, during covid, it was not. Get a list if requirements from your LOCAL office and see what you can work out. Good luck.