Highway Tolls
- Every car rental in Austria will come with the highway vignette. If you are unsure ask at the rental counter.
- Slovenia also has highway vignettes that are linked to the car's license plate. You can purchase this online once you have your rental car. Be sure to purchase this before you enter a Slovenian highway. The current cost is 16 euros for one week. These can also be purchased at gas stations, but I've only done it online.
https://www.gov.si/en/topics/tolls/
- Croatia has a pay as you go system with toll collection at the exit. Credit & debit cards are accepted.
Zone Surcharge for leaving Austria
Depending on the car rental company, there is a surcharge for leaving Austria. For example, Hertz will charge about 18 euros per day for the entire rental (not just the days you are across the border). Based on my experience, some Hertz locations will ask and add the charge and some will not. For a longer rental, this adds up. This information isn't obvious anywhere on the Hertz online reservation system.
Sixt has a flat fee of about $60 this year that you can add to your rental when you make your reservation.
Driver's Permit
As others mentioned the International Driver's Permit (from AAA in the US) is an official translation and verification that your driver's license is valid. You will still need to bring your driver's license. The rental companies don't really care about the IDP, but if you are stopped by local police, the police may make a fuss, it depends on the country and the officer. AAA charges about $35 (IDP and photo). It is valid for 1 year and you choose the date it becomes valid when it's issued.
To avoid fines, make sure you know the basic road signs and traffic rules in each country. For example, in Austria, when you enter a village (there is a sign with the village name), the speed limit drops to 50km. You are expected to know this and there are no speed limit signs. Drive the speed limit even if everyone else is not ,as there are plenty of speed cameras. No right on red and no passing cars on the right.
Border Crossings
Generally there won't be any border crossing stops between these countries. However, we noticed some enforcement on small back roads in Austria, both along the Hungarian and Slovenian borders last year. They will ask for your passports and send you on your way. We did not see any checks on major highways.
Enjoy your trip!