Four of us will be driving the countries of Croatia, Montenegro, Bosnia & Herzegovina and Slovenia. Can anyone give me as much advice as possible with regard to handling the tolls please?

No tolls in Slovenia. Instead, you need to buy a driving pass called a vignette that you attach to your windshield. It is required to drive on Slovenian highways. You can buy a vignette at the border (both sides) as you enter Slovenia. If you rent the car in Slovenia to start, it will probably come with a vignette included.

In Croatia and Bosnia, just pay the tolls as needed. I used my US credit card to pay tolls in Croatia. Don't remember many tolls in Bosnia, maybe one near Sarajevo. Get some local currency and just pay as you go.

Thank you.

If you are renting a car, you must inform the clerk of the countries in which you intend to drive, as some of them have "cross border" fees for various reasons; and some will not permit you to take a rental into certain countries.

Thanks. Yes. That fee is actually Euro 8.75 per day. I am permitted to cross borders.