My one recommendation is to make sure your pickup times for the reservations is for *after* the latest time you'd be picking up the car. That way, you can easily cancel the reservation you don't need in the National app or online. IME that requires a phone call after the pickup time.

I always like to make sure the reservation I'm not using is cancelled before I drive off the lot. If the booth agent sees you in a van, they may assume the van reservation is correct and charge accordingly, even if it's on the aisle.