Just a FYI...Xtabi does post their prices...they have menus placed along the bar as soon as you arrive.
And, just another thought not a judgement...but maybe the bartender at Xtabi simply made a bad judgement call when he was really busy. We've watched so many tourists say to a bartender "keep the change" after he's taken the time to calculate it all out, that he may have assumed this was going to be the same scenario on such a small amount.
To each his own reaction if you feel screwed over...but maybe this is simply a bad judgement call not anything more!