How to calculate comission for staff?

Hello to everyone
I have the following situation:
In the bar any customer can buy a drink for the bar-girl (we call it lady-drink). For example, the price of any lady-drink - 10$. At the end of the day each girl gets 3$ from each purchase her a lady-drink.
I have added to each lady-drink a list of girls names as order tags. So, at the end of the day I can see how much drink was purchased for each of the girls (Mary - 5, Ann - 6…)
I would be very grateful if someone explain to me how to make an AUTOMATIC CALCULATION of girls comissions in the end of day (Mary - 53$, Ann - 63$…). And most important how to transfer the money from the cash drawer to the accounts of the girls AUTOMATICALLY.
Thanks in advance)