I actually have first hand experience on this.
They sell a small bottle(like 1oz) of hard liquor for $4 or $5. Get a free coke and you have yourself rum and coke all you want.
On a recent flight....a friend of mine(who is obnoxious when hes drunk) was doing this and I couldn't stand it anymore.
So I told the flight attentant that he already had 4 drinks at the airport and 2 here so they shouldn't serve him anymore.
But they didn't listen to me and kept on serving him.
Their reasoning is that he didn't look intoxicated.
So to answer your question....order as much as you want as long as you pretend you're fine
