Well, today I looked it up on Expedia. I made a reservation at $18.99 a day, but I was charged $58.99 a day. I called Enterprise, and they said that because it was a reservation instead of prepaid, I had to pay the regular rate.
1) Fuck Enterprise for not honoring their advertised rates.
2) Fuck Expedia for showing that rate even though the reservation didn't require the agency to honor it. The small print says that the amount listed will be "charged at the time of rental." I'm not even sure it's possible to prepay — I didn't see that as an option anywhere. Maybe it's just purely deceptive advertising. The rate was a lot lower than other agencies at the DFW airport.
I figure I'll wait a week and then write a letter to both companies, letting them know why they have lost my business.