I'm not 100% sure how it works in Thailand, but many surgeons require some kind of consultation prior to scheduling surgery. So you'd have an opportunity to talk to them and ask questions then. In the US, this usually costs $100-200.
In general, if not paying with insurance, a full payment is expected prior to service being completed. My surgeon required a 10% deposit to secure a surgery date, and then full payment one month prior to the date. This had to be done via wire transfer from my bank account.