Cross Border card for rental car

I suspect it may be a green card, which is an insurance paper that makes it possible to drive a EU car in non-EU countries without buying additional insurance, so it would only apply to visiting Bosnia, since Slovenia is in the EU and Croatian car insurance is automatically recognized there. If that is what they are offering you're better off buying it instead of having the hassle of buying extra insurance for Bosnia at the border. It's probably best you contact the company directly and clarify what it is. On a not so side note, tt could potentially also be a vignette (toll sticker) for Slovenian highways, but the description is too vague so I don't think that's what it is. Still, since you will be driving in Slovenia, don't forget to buy one and affix it to your windshield. It costs €15 for a week and you can buy it at major gas stations in Croatia near the border and at all gas stations in Slovenia. I wouldn't ask the rental company to provide it, since they will probably charge you extra for it.