If you wish to cross to another country with the rented vehicle please ensure you read the ACROSS THE BORDER RENTALS segment from the T&C (Terms and Conditions) of the offer of your interest. There you will see the list of countries that you are allowed to cross with the rented vehicle. Please note that if you are allowed to cross to Estonia and Lithuania you will have to add a Cross Border Card to your booking on OPTIONAL EXTRAS.

I suggest you call them or write a quick email to them. They typically answer within a day or two