I had that happen to me several years ago, when the law made these tickets unenforceable around the time that guy who found out he had cancer was ticketed.
I was 15 minutes late, got a ticket with a ridiculous admin fee, then a "reminder" with another ridiculous fee attached. I sent a registered letter to the Impark president with 2 loonies and 2 quarters in the envelope (the difference between what I paid and the time I used) along with my opinion of his company's tactics and a promise I would go to the media and see him in court.
I never got a response or another bill. My last credit report was perfect, like normal.
Keep fighting these guys - they put in meters to cut costs of having a teller on site, but they have no problem paying more expensive security guards to write tickets. Just one of many cash grabs in this city! In any other parking lot, I would have been charged the difference before leaving, which I had no problem paying.
If I recall correctly, the law may have been reversed, but I am sick of people trying to fuck us over. Things happen, people are late. We owe the difference, fine, but why screw us over?
To this day, I have never parked at an Impark lot since - I will walk three extra blocks or more before I ever give those pricks another dime.