A USURER TO BEAT THEM ALL

I had an unpleasant experience with another usurer to beat them all.
I signed up for a Coles Myer Master card a few months ago principally for the petrol discount vouchers paying the monthly due amount each month except last month when the monthly payment was credited to another company, the result of an Internet banking error.
When I discovered this I immediately paid the total amount due on the day following the due date.
I subsequently discovered that I was charged $9.17 interest, an interest rate charge of 1.8229% for one day eg 667.6455% per annum.
This contrasts with the obligatory statement what their interest charge is which they stated at 18.5% per annum.
Obviously they use a different calculator.
To make matters worse instead of the banks who take a day to process their payments (overnight processing) several days later I'm still waiting to see the payment credited to my account. Not only that, the poorly designed web account is not secure either. Perhaps problems like this have caused their consumer loss of confidence.