The package will cover 15 alcoholic drinks (Plus is $15 or under; Premier is $20 or under). If you end up drinking more than 15, then you will be charge the additional drinks. This often happens when the crew add two drinks to the same person rather than to each account holder.
Additionally, if the price of your drink is higher than what the package covers, you pay the difference plus they add 18% gratuity to the amount it went over.