According to Philip J. Fry, Bender knows how to make Ice Cream Soup. If we're assuming there's actually such thing as ice cream soup, then it is a bowl of melted ice cream.
Also shakes aren't completely melted. Ice cream soup is like you got yourself a bowl of ice cream then let it sit for a half hour before you touch it. Or put it in the microwave long enough to have the same consistency as milk (which shakes are thicker than).
Round up the tip.
I think the only thing that would make me not tip is if I've been sitting for like 20 minutes after I've been seated and I still haven't been brought a menu or a drink, or if it has been 20 minutes since I've been brought the menu and the server hasn't come to take my order yet.
Oh, and also, 200 HUNDRED DOLLAHS FOR A TIP CALCULATUH!