TIP means "To Insure Promptness." But that extends its meaning to quality. 12% is more than enough for delivering cups, glasses, plates and/or bowls with promptness and clean courtesy. However, it is a gratuity, which means a free gift by the patron if the patron so chooses.

For those patting themselves on the back b/c they are big tippers, go crazy and tip 50%, big shots. It will simply make matters worse by promoting a dubious entitlement for carrying plates and utensils.