Princess have made this complicated.
If you buy the Princess Plus package, you will pay $60 per day. This package includes the Plus Beverage package. Similarly, if you buy the Premier Package, at $80 per day, the Premier Beverage package is included.
Logically, therefore, it makes no sense to buy either of the stand alone drinks packages BUT, the Plus and Premier packages have to be purchased by both adults travelling together. If one adult does not drink alcohol, and drinks little in the way of soft drinks, but the other adult enjoys a few drinks, then economically it just about makes sense for that individual to purchase a stand alone drink package.
Hope that sort of makes sense.