if you are renting the car from hertz, they shouldn't charge you for the shuttle ride between their office and the train station (not sure if they would pick up your entire group or just you and then you would have to drive back for them in the rented vehicle.)
also, when i looked at the mears website, they only mention transport between the airport and WDW (or other orlando attractions), not the train station. they may have this service, but as it is far less popular, you may have to call to confirm that they have it and get a cost.