Looking to rent a car for an eventual roadtrip in the Balkans to countries such as Serbia, Montenegro, Bosnia, Croatia, Slovenia as well as Bulgaria and Romania. Does anyone have experience with going crossborder with Sixt in these countries?

The best price and policies seem to be for Sixt in Zagreb and Ljubljana. However, the website says that crossborder travel must be approved by the local Sixt office. Is this a sure thing or will it be a hassle? Thanks.