The tour, through Viator, will pick us up from where ever we choose, hotel, port, etc. Because there will be 4 of us each with a check and a carry on bag, it will not likely fit in their tour van/shuttle as it is not a private tour.
And it won't all fit into a rental car unless we get something extra large in which case it becomes cost prohibitive, even if we drove it up to Venice vs taking the train. Even if we could squeeze it all in, I think we would want to store the luggage at the train station while we drove tot eh castle/cave anyway as I agree that leaving the luggage exposed in a rental car does not seem wise.
The tour I am looking at is through Viator. I have been to the castle before but the others I am traveling with have not. I love Slovenia and so wanted to give them the opportunity to see another country although not very far into it. We thought it would be a nice excursion before going up to Venice.