关于美国预订supershuttle小费问题

各位童鞋，最近在supershuttle上预订机场-酒店往返接送，在最后付款栏上提及“小费”问题，本来打算先填“零”（网站也允许），见师傅后再现场给，殊不知系统一直提示“Gratuity to Add: (0 for no gratuity)$
Please enter gratuity amount if you'd like to include the gratuity in the payment amount. If you do not wish to include a gratuity at this time, please enter 0.
Please enter a numeric value zero or greater for the driver gratuity.
The gratuity cannot exceed the total fare. Please enter a value which is less than the total fare.”，无法到下一界面，请大侠们指教！感谢！