We have a reservation with Hertz to rent a car in Dubrovnik and drop it off three weeks later in Ljubljana. The USA Hertz reservation line says that basic required personal liability is included, but they can not answer my question of the insurance is just for Croatia or for Croatia and Slovenia. The State Dept website advises that one must often purchase liability insurance at the Slovenian border. I would like to get everything at the Hertz desk in Dubrovnik. Has anyone had experience with this? Thank you!
If Hertz can't answer this question to your satisfaction, rent the car elsewhere. Renting a car in one country and driving it into others is extremely common, so it's not like you're asking a difficult question. Slovenia isn't exactly a third-world country, either.
I suspect a big company like Hertz would be able to offer you insurance for multiple countries, given that you can rent a car from Hertz in Slovenia and be insured for driving in Slovenia...