We can recommend Oryx if you plan to rent a car in Croatia. We took a train from Ljubljana to Rijeka and it was a pretty easy walk to the port to the Oryx office. We rented a very nice, new little car (only 1,200 kilometers on it) along with a Wifi Hotspot. We let them know we would be driving to Mostar and Kotor so that we got the right paperwork for the car, and we did, indeed, have to show this at the borders. The Wifi worked beautifully and I used my cell phone for GPS, but they made sure we understood it would only work in Croatia, and that was accurate. We returned to the Dubrovnik airport with no issues other than that they were swamped with cars and parking was a bit tricky. Total cost for everything including Wifi, cross border fee etc... was 3,100 kuna or $486 which was exactly the estimate at the time I reserved - no surprises.
In that 10 days, we did pay for some parking fees and tolls, and there were 3 days that we did not touch the car - one in Spilt, one in Dubrovnik and one in Kotor. The driver was very glad to get three days of rest! But for driving around Istria, to and from Plitvice National Park, down the coast from Split to Dubrovnik, and to and from Mostar and Kotor, we though this was well worth it. We didn't want the hassle of returning and picking up cars multiple times.