I remember a while back that I too had booked a minivan when a couple of friends came to Florida with us.
At the time Dollar had no 7 seaters available so they gave us a 12 [or maybe 15 ?] seater without any extra charge.
I absolutely hated it though as it had the turning circle of an oil tanker and parking it in confined spaces was a nightmare!
We swapped over to a car after a couple of days because our friends were to return to the UK a week before us so we didn`t really need the extra luggage carrying capacity that we had on the day of our joint arrival.