I don't think xs charges tax on the drink. I haven't been a serious clubber for about 10 years. 10 years ago, a $2 tip per drink was a solid standard tip for a walk up bar at a club.
I'm guessing that this is still the case at most clubs in most cities. After all you're paying $2 for < 2 minutes work so >$1/minute which is very fair.
However, XS drinks are priced at a premium over the typical drink at a walk up bar so I could see how a bartender would expect a premium tip.
* If a typical drink at a walk up bar is priced at $10 then a $2 tip would represent 20%.
* A 20% tip on a $14 drink would be $2.80
* A 20% tip on a $17 drink @ XS would be $3.40
Therefore I think bartenders @ XS would expect a grownup to tip at least $3 b/c you're at a premium club paying for drinks priced at a premium. A $2 tip would be perceived as lowball.
Since the total price for 2 drinks @ XS is the cost of a meal I could see how a bartender could expect a tip to be the equivalent of the minimum tip that one would pay for a meal. In this case any tip less than $5 would be perceived as lowball.
Also, since you're not doubling their work with 2 drinks (only one customer request, only one cash transaction) I could see how a $5 tip for 2 drinks would pass muster. I only ask b/c some bartenders seemed to be off-put by a $2 tip per drink based on their body language. So I think $3 tip for 1 drink, $5 tip for 2 drinks seems to be the sweet spot @ xs.