If anyone want to take my advice, visit that spa 3 to 4 times over a period of time, do not take FS. B2B for not more than 2k.
You'll get FS in your 4th or 5th visit in under 3k.
I've got FS in my 3rd visit for 2.5k but that was 4 years ago. Counter, I pay strictly 1500 only.
Over my subsequent visits, I've got services which Bangkok would also shy away from.