I did that once. It was exact change only and I didnt have 50 cents. There was a bunch of people waiting behind me (it was an exit) so I got out of the car and stuffed a dollar bill in there. After seeing a thread like this a long time ago, I was hoping for a check in the mail for the 50 cents more I paid

Maybe they kept it because I'm pretty sure I clogged the hole for the coins!

+1 for sunpass though