If you have a Thai bank account the amount and charges vary according to the bank. I have a Kbank account. 30,000 baht is the limit for a single card less withdrawal (using my mobile phone) there is no charge and I can withdraw multiple times.
if you already have the 12 months extension then you can renew it elsewhere. I got my first one in Pattaya and renewed in Chumphon with an updated TM30
Highly recommended, can give you an estimate and written quote at first consultation. Been there many times. Speak English fluently and will pick you up from and return you to your hotel. They are in Sukhumvit