That's a story I'd love to read about. OnTrac seems to use basically junker cheaply painted white mid-90s minivans whenever I see them about. They simply slap a magnet with OnTrac on it and it becomes a fleet vehicle.
While FedEx/UPS have their own custom designed fleet, OnTrac seems to scour auto auctions for theirs.
edit: They basically leverage their amazon fresh / amazon now deliveries.