It was apparently a XTS Cadillac, according to the email that I received from the manager a couple of days ago! It was quite nice to drive around and not too bad on fuel.
Time to give an update on what happened. I would gladly post the manager's email here (minus names, of course) but I'm not sure if it's ok to do that. So I'll summarize it. I think the manager's native language must be Spanish, because his English is a bit weird in places. But the gist of it was that he understood that there was a misunderstanding at my end concerning the upgrade. He said he would forward $500 "to our accounts department" as a courtesy. He said that once I accept and sign for an upgrade then the signature "ensures us that the business transaction was acknowledged". He would have refunded me the upgrade if it was just a one class upgrade. But the the price difference between a full size car and the XTS Cadillac is $35,000, apparently (surely that refers to the cost of the vehicle).
I am happy to accept because I don't think that I am going to get anything much better than that. If I continue to pursue it, the cost of the hours and energy I spend on it will far outweigh the 200 dollars that I didn't get back. I wasn't informed about the cost of the upgrade, but I don't see any advantage in continuing to insist upon that.
Now all I have to do, apart from wait for the $500 refund, is wait for the Florida scam rip-off toll charges to start bouncing in.