If you don't have a drink package.....
Every drink has 20% added to it as a "gratuity/service charge". A $10 drink will cost you $12.
If you do have the package via promo...
NCL covers the cost of the package ($109 per person per day) but you will pay the 20% ($21.80 pppd) of the package cost before you board because it is included in your final payment cost. With the package, the $10 drink is completely covered and the 20% that you would otherwise be charged is covered by the 20% that you already paid. Even if you have twenty $10 drinks in one day, you won't be charged anything additional. Your package covers included drinks up to $15. If you get a drink that is over $15 ($20, for ex.), you will pay the difference ($5) plus 20% on that difference ($1). Your $20 drink will cost you $6 onboard. I'm using U$D just to get the point across.