I've done it couple of times here in Canada HLE locations because of the cheapest weekly rate (not always). Normally they will call me 1 day before and ask if I am okay with a regular car since all cargo vans are rented by those courier companies. It's interesting to see that they never close the availability online and still allow people to book.
HLE manager is pretty nice and he figures out after my 5th "cargo van" rental that I do not actually need the van, normally he will give me a mid-size or compact suv.

1, Most cc do not cover cargo vans or even pick up trucks, call cc company and ask them to email or mail you latest T&C of CDW, couple things may have changed since COVID.
2, It's always YMMV and you may be able to switch at another HLE location (whoever wants the cargo van). I think it's better to book another reservation at nearby location as back up (If you book at same location they may want you to keep the more expensive one if they figure out that you don't want the cargo van).
Call the location 1 day before pick-up to check what cars they have (may not work at some locations)
3, If you book a postpaid rsvn, you can cancel free of charge at anytime before picking up (should be able to cancel at anytime but it's better to do it before picking up), so simply get there 15-20 mins earlier before pick up and see what car they have for you. If they actually have a cargo van available, you can check for alternatives or cancel and walk away.