Glad to see this post...reminded me that I need to put some money in the mail to the Florida Toll Authority (or whatever it's called)!
We went from the port straight to WDW and hit a booth or two that had no worker and required correct change -- i.e., coins. We had already used our change at prior booths and only had dollar bills. One booth had a sign that said that if you found yourself without correct change, please take an envelope and mail the correct fee back to them. Another booth had no such sign, so I figured I'd just add that fee to the other envelope with a note to explain. I could not have been more frustrated, as I had plenty of money but no way to pay the toll properly.
In summary, take at least $5.00 in quarters and few extra dollar bills to be safe....and use the paper money as much as possible to ensure that you have correct coinage available when required. Otherwise, you'll end up setting off the alarm more than once -- trust me!