Sorry to hear lomo. experience.
It is very rare to have "additional charge" if you book the room in advance. It only happens to those taxi-referral since the sna/msg parlors would give "introduction fee" to taxi drivers if they bring in new customers, which is quite common on BKK.
While this crazy thing may happen in China, I have one more guess. This "oil massage" service/session is managed by another team of HS (like 威尼X). Ms. Lee (your manager) is not managing this session and she does not get any commission from the booking of this

il massage:, thus giving you a higher charge. In that case, get the oil massage booking directly with those managers, not thru. Ms. Lee.
Anyway, it is unacceptable to have this kind of thing.