13 cents debt becomes R78.63 overnight

I object to E tolls for a number of reasons but pay as a casual user since we don't use the highways much and I'm a law abiding citizen. I visit a Sanral centre within 7 days of using the highway and pay cash for the tolls.<br> When I made my last visit on Sept 3 2015 I couldn't understand my statement. Somehow the previous payment I had made was recorded as 31 cents short. Since the discount date had now passed, the 31 cents had become R78.63 payable. I tried to argue this couldn't be right and got a supervisor involved. They didn't explain it at all, basically saying that's how it is. I have paid it but the point is this kind of penalty must be *******. How can a 31 cents short payment suddenly revert to the initial larger penalty amount, ignoring the amount I'd already paid? It must be contrary to the Usury Act.