The additional fee of $20/30 is suppose to cover the gratuity as well as the upcharge. However, we usually have outstanding service (such as our Jan. Millie cruise) and do tip $10 extra.
The only time that didn't happen was on the Mariner when I was furious with the service. Because we were paying $50 each for the Murder Mystery, and where they sat us the idiot who waited on us was constantly asking me to move this way and that so the actors could get by!!! Needless to say he received nothing extra.