Here's a google translation of an advert for bar boys.
"Fun Size Bar
Now hiring. Applications are now open.
413/45, village No. 12, Nong Prue, Subdistrict, Bang Lamung District, Chonburi 20150
job requirements Bright smile and positive attitude, gay man homosexual woman or transgender person
Barboy Offer (Full Time) Full-time Barboy 150 baht per day, 60 baht for drinks, 1 day off during the week, 7 days of unpaid work and 1 day off per month to take any day off as needed. (Must notify manager 24 hours in advance) Barboys still get paid for the day and they are off 150 baht from 6:00 p.m. to 2:00 a.m."
I think 50 or 60 baht per drink is standard in Jomtien.
There appears to be an additional daily payment of 150 baht.
I was under the impression that they only get commission on drinks at some other bars.