I see
this article with the statement "The charge is expected to be added to the cost of arriving in the city by either train, cruise ship, bus or plane." I'm not sure if that refers to those starting in 2020. I'm first reading it as meaning the charge will be added to the train fare, etc., and will the hotel stay tax be reduced by that amount? Then there are several logistical questions, such as: if this is part of the train fare when booking on Trenitalia, how do residents, etc., show that they're exempt? By plane, many people arriving at VCE airport may not be going into Venice; maybe the charge will be added to transportation into the island city from the airport.