today some users found out, that it is possible to book DB-ÖBB fee for the Italian part also via Czech Railways CD.cz, you pay in second class 251 CZK wich is about 10€. Via CD the reservation is included in the price, via ÖBB not.
You have to search for a train and add as discount Interrail or Eurail Pass.
It also works for RJ via Tarvisio Vienna-Venice, but it shows a wrong price of 88 CZK wich is about 3,50€, but there it is also the fee 10€.
If on board of the train the train staff will accept it or not is not known at the moment.