Sounds like it's a mistake fare, but you've got it confirmed.
I'd be leery of relying on this, since you may show up to find that they won't honor it (just as airlines and hotels don't always honor what are obviously mistakes).
Booking a backup "normal" reservation at Dollar might be a bit tacky (though not out of the question), but I'd have a backup at least at another agency you can retreat to if the price they "fix" the mistake to is too high.