Update: Finally heard back from them. The good news is I'm hired, and start in two weeks!
The bad news is that they will not be covering mileage. For the first two months, they want me to commute to a location that's roughly 60 miles away from the location I'm being hired at 4 times a week for the first two months, to cover for someone going on leave there, and then supposedly travel less once that period is over, though still as needed.
Rather than pay for mileage, they've told me they're giving me a 50 cent increase in pay from what I was offered.
Now, this actually doesn't seem like a bad compromise, since I'll be making more than I would have before once this two month period is over. However, should they ask me to travel more in the future, would my agreeing to this make me unable to ask for compensation should they ask me to travel further?