I want to hire a car in the beginning of January 2019 in Barcelona airport. I want to travel abroad to Andorra (possibly to France) for few days - should I pay additional charge for this?
No, there is no additional cross border fee.
Great, where can I read the conditions about it online?
Look here to be sure:-
To view the CROSS BORDER or FERRY terms and conditions. At STEP2 of our 3 step online booking process, alongside the car photo of your choice, there is a link entitled “IMPORTANT INFORMATION”. Clicking on this link will open an additional smaller window, with the terms and conditions appertaining to that particular car/supplier. Look under TRAVEL RESTRICTIONS
Try a few different suppliers as each supplier/vehicle type may have a different cross border rule.
I can read it only after I made a booking, correct?