They didn't charge me extra for it in the end. Returned the car a couple of hours late by mistake, but they said it was not a big deal.
Car Hire Zadar Deals - Sixt rent a car
Sixt has two locations here in Zadar, and you can hire a car from either. Our office in Zadar Airport is useful if you want to receive your vehicle soon after your flight lands, and is conveniently located in the local municipality of Zemunik Donji for quick access. Zadar city is close by, as are other tourist towns and villages such as Petr Dane. Our city centre station is not far from Zadar's major hotels and holiday apartments, and is convenient for all the tourist attractions the city has to offer such as its Old Town.