Unsure on your credit card question; best to check the terms on the website or call them. As for the van, I've rented a cargo van twice from an HLE when I've needed to pick up some larger items. The first time I showed up they couldn't get a cargo van, supposedly they were all rented by Amazon/FedEx/etc for package deliveries. They offered me a minivan in it's place and it served me fine with the seats folded. The second time they had the cargo van; it was a Ford Transit with power locks and windows and backup camera as the only options. Worked out great because I needed the room but it was not something I would like to drive 2,000 miles in.

I highly doubt if you end up with a cargo van that they will let you switch to anything else, especially at an airport. Airports don't carry the cargo vans only the 15 passenger variety, and even those are rare. Same for HLEs; they only order them from distribution when there's a reservation for one, so they don't want to be stuck with a van they can't rent out to anyone else. You can book the van and pray the day you show up they don't have one and offer you a minivan or something else. But with inventory as tight as it is, I wouldn't be surprised if they called you a few days before to confirm you're coming for the cargo van they'll have for you.