We indeed got a Mercedes ML250. We chose the rental and return location at Frankfurt Central Station. The rental office is inside the train station. We met a kind German who accompanied us all the way to the office. The car is in the underground parking garage P3 of the train station, and the pick-up and drop-off are at the same place. We were worried about additional fees, but today we received the charge notification, which matches the order amount, so we are relieved. It's worth noting that the rental office doesn't inspect the car with you in person, so we were worried something might come up after returning home. Fortunately, all was well.