The menu at Bangs Lake Bistro states "A gratuity of 18 percent will be added to all bills for parties of six or more." The restaurant distributes this amount to all serving staff, regardless of whether they had actually served that customer.
Isabella's bill for her party of eight included an amount on the "tip line" equal to 18 percent of the price for food and beverages. Because the amount was dictated by the restaurant's policy and not Isabella's opinion of the service received, the payment was not free from compulsion. The fact the amount was labeled a tip did not make it one: It was a service charge. (However, if Isabella had added an extra three percent to the amount because the service was extraordinary, then that three percent would be considered a tip.)