We plan to rent a car in Zagreb, drive throughout Slovenia then back to Croatia to return it in Dubrovnik (2 weeks time). Is there a company that has seemed fair and trustworthy? I plan to tell them about the visit to Slovenia but I read a number of concerning things online about a couple companies. Also, what is required or smart to have regarding insurance? I am checking with my credit card company, but I do not believe it covers international car rental.
I usually start with a price quote from Auto Europe and they work with several European suppliers. You do need to tell any company which countries you'll visit and on Auto Europe's web site, I believe this means that an online booking needs an agent to follow up before finalizing it.
You probably do need to purchase Collision Damage Waiver insurance. Check the deductible level associated with that ($500 or more?); they're usually willing to sell you extra insurance, bringing it down to "zero deductible." See Rick's run-down on the insurance issue: http://www.ricksteves.com/travel-tips/transportation/car-rental-cdw.
We often use economycarrentals.com. They also work with many suppliers and you don't learn the name of the company until you have accepted their quote. We ended up with a company called Last Minute Car Rentals, which didn't inspire much confidence, but everything was normal and we had no problem. We also drove out of and returned to Croatia. We checked the box on the on-line form and the car came with all the necessary papers. The insurance included full coverage. There was a deductible of about $650 but it was refundable by Economy Car Rentals. Happily we did not need to use the insurance so can't comment on it.
I've made a couple of car rental reservations with EconomyCarRentals recently - haven't actually picked up/used the cars yet. FYI, there no longer seems to be a "box" to check when reserving the car saying you want to take the car out of Croatia. This may be because Croatia is now in the EU. I'd think there would be no extra restriction/cost for driving such a car to Slovenia, also in the EU. However, I want to drive my rental to Bosnia and Montenegro (not in the EU). I've contacted a few Croatian car rental companies and have been told it's anywhere from 30 to 40 Euros extra (plus 25% tax) to drive a car out of the EU - but again, Slovenia wouldn't apply.
EconomyCarRentals reservations are refundable til 3 days before your rental. They charge you a small up-front fee - a deposit - that is also refundable. I have canceled/refunded two of my reservations and the deposit was duly credited back (though I lost some money because they charged/refunded in Euros, and the Euro has fallen since my reservations were first made). Anyway, you are free to make your reservation with them, see who the actual car company is, contact them about potential extra costs, and then keep it or cancel it before 3 days before the rental if you find someone better.
I have used American Express's Premium Rental Car coverage in the past - costs $24.95 per rental ($0 deductible). It's free to sign up, charged only when you use your card to pay for the car. I will call them again to confirm it will cover my car in these other countries. The Croatian car companies I have been checking with say they take Amex as payment (not everyone in Europe does).
October 2015-We traveled from Zagreb to Slovenia then to Dubrovnik in a rented a car thru AutoEurope. They set us up with Fleet Car Rental. We were to told up front and expected a E60 fee for taking the car into Slovenia which we paid to Fleet in Zagreb. Also, once you enter Slovenia, pay for and pick up your Vignette, a pass to ride on limited access highways in Slovenia. A week pass was E30, still much cheaper than the tolls in Croatia which are very high. When you pick up your rental you should check the windshield to see if any unexpired days on the Vignette are available. You most likely will get ticketed if you do not have a Vignette.
Check with your credit card company, mine, Capital One, advised to not purchase any extra insurance and if we did they did not cover any claims whatsoever.