FWIW, I've been using Costco (costcotravel.com) to book my last few rentals. MUCH better pricing than even our corporate rates. It was a similar situation to what you're seeing where the rental car agencies were either sold out or were charging $120+/day. Costco ended up being 1/5th that.
For all of my rentals, a second driver & unlimited miles were included free. Their site says this may not always be the case, but it has for me. For all of their reservations, no cancellation fees. Bonus: You can use any credit card you wish to pay for the rental. Not just Amex.