ご参考

スロベニアのレンタカーには高速道路利用の前払ステッカー(vignette)が
貼付されていますが､クロアチアのレンタカーはどうでしょうか。国境近くのＧＳで購入する必要があり､失念し捕捉されれば高額の罰金が科せられます。
http://www.si.emb-japan.go.jp/website_jp/doc/ryoji/vinjeta2015.pdf#search=%27vignette+%E3%82%B9%E3%83%AD%E3%83%99%E3%83%8B%E3%82%A2%27
クロアチアのレンタカーのDriving restrictionはハーツのご利用条件が参考になります。高速料金は非常に安い。
Hertz vehicles cannot be driven into and/or dropped off in any African, Asian, Middle-East countries, any island, nor across the borders of or beyond the following countries: Albania, Armenia, Azerbaijan, Belarus, Estonia, Finland, Georgia, Ireland, Kyrgyzstan, Latvia, Lithuania, Moldova, Russia, Turkey, and Ukraine.

A cross-border fee of € 8.75 (including tax) per day with a maximum of € 26.25 (including tax) per rental (for rentals up to 30 days) will apply for any vehicle taken across the border, except for the Croatian-Bosnia Herzegovina border on the A8 Main Road (Route E65) between Split and Dubrovnik, if less than two hours are spent in Bosnia Herzegovina.

Should these restrictions be ignored all insurances (Third Party Liability, CDW, TP, PAI, Super CDW), if accepted, shall be deemed, null and void. The driver will be held fully responsible in case of damage to, or loss of the vehicle and will bear all consequences, including the cost of repatriation of the damaged vehicle.