I'm a US citizen driving from Bulgaria to Greece in a rented car. Where can I cross the border and what paperwork will I need?
As you are not the owner of the car, you will need a letter of authorization from the owner. Please contact the rental company for it and eypect to pay an additional fee for it.
Which border checkpoint to take depends on your starting point and your destination. The major one is Kulata - Promachonas.