Carnival does add the 15% to any alcohol you buy.
If you only go to the Horizon Court once, you'll think it's a zoo. You have to learn how the Horizon works before you can appreciate it. Aboard Carnival, and all the other lines I've sailed, the buffet line is just that,,,,a line, but at the Horizon Court, you're free to just walk up to whatever you want, and serve yourself. ie: you don't have to stand in line behind the lady who can't make up her mind about how many croutons she wants, in order to get some strawberrys. Just walk up to the strawberrys, and serve yourself. This actually works very well, once you get the hang of it. Sometimes, there are a few people who all want exactly what you want,,,but honestly, not often. We've always enjoyed Princess, and will continue to sail the line, when the price is right.