It sounds from your email like you didn't have a copy of your reservation with you. I'm not saying that that makes what happened right - but at least if you had a copy of the reservation you would have had a leg to stand on - they would have had to give you the Volvo at the rate you booked.

What I would do at this point is go back and review your original reservation. If you did indeed correctly book the car, I would write to Hertz through hertz.com and give them the details of what happened, and tell them you want a refund for the extra cost of the Volvo. My experience is that Hertz is generally very responsive when you write with a complaint.