Never rent a car from a Serbian AVIS (or a global one either).
The situation is exactly the same as everyone else's. When receiving the car, they checked the card on some website, they said it wasn't a credit card (although there had been no problems in neighboring offices before), and therefore they had to pay 39 euros per day for extended insurance, which is 4 times the cost of renting the car itself.
And you also have to pay for the "premium location", which, of course, was not mentioned at the time of booking. It's not clear what the premium location is, I've never seen it anywhere else, and for this money you can order a car anywhere in the city with delivery.