FWIW, I seem to have gotten undercharged by National for my tolls. They billed me $10.50. I had a rental car for 4 days, and I went through pay-by-plate tolls on at least 3 days. The website says I'd be charged $3.95 per day, noting it's a 24-hour interval. Even if I somehow got "2 days for 1," I'm sure I went through at least 10 tolls, and the average charge had to be close to a buck.
I hope others have similar luck.

I don't know what you do if you're overcharged, though.