We do have a Gratuity option in BO, and a service fee option for Online ordering.

This can be done based on custom development. 

it will be available with Linga Driver app. The app will be available by end of this week. 

https://www.lingaros.com/restaurant-operating-system/linga-driver/

This was fixed after the new pax file update. 

Hey Jesse, it is possible in a conversational mode, when you unselect the modifier it turns to "No", please test it.