We want to book a rental car at the Airport of Frankfurt and travel to Romania and back, do you allow?
I see you have conducted a search and have viewed the available cars.
Just use the website, you can check here:-
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. Choose then the CAR tab and look under TRAVEL RESTRICTIONS
Try a few different suppliers as each supplier/vehicle type may have a different cross border rule.
It seems only Flizzr and Sixt allow cross border from Germany to Romania based on your search criteria.