Re:Tips are more than appreciated; they are REQUIRED --Or Else
Usually, the fee is called a "service charge" rather than a tip, which would have solved matters better. Apparently, it was called a "gratuity", which is, by definition, supposed to be voluntary.
I come from a family of reasonably generous tippers; when there is a service charge added that is less than 20%, I typically add on to it to make it 20% or, if the service is exceptionally good, 25%. Some of my family members, on the other hand, will not give more that the service charge, even if they would otherwise have tipped more.