How do you prove that you paid the toll if you don't have a receipt?
The ones we experienced that aren't manned (ever) have a green and red light that change when you've paid. It also counts down the amount you've put in.
One of them on our last trip was really funky, and we ended up paying twice, LOL. The first time it just didn't look like it had recorded it, so I got out my camera for the second attempt, and took pictures as it counted down the quarters we put in. Did it just to prove we'd paid - it never said Zero...it just went back to the total price the next person owed. The light, from memory, was totally broken, so that was of no help!
But those unmanned ones have handy envelopes to take, so if you run into one and don't have change, just snag an envelope and send your money in.
DH's ridiculous GPS put him onto a toll road one night. I still don't know how this happened, as he was going from Old Key West to the Publix across from the Super Target, and it was a pretty straight forward, surface-street drive, that I'd written directions out for (and he'd BEEN THERE!), but he believed the GPS over me, and found himself going through an unmanned 50 cent toll with no change whatsoever.
Since he got the envelope, I tossed a check for $1 (I just couldn't write a check for 50 cents) in, filled out the envelope (you have to put time, date, your license plate number, so make sure that part is filled out before you drop off your rental car!), and had my local aunt mail it in. Could hardly have been easier!