Truworths Interest Charges

When I paid my Truworths account this month I made a typo and paid R80 instead of R90. So I'm in arrears for R10. Because of this R10 I have received 3 sms's and 2 emails to warn me to please pay this R10 immediately. I have no problem with this except the fact that for this R10 arrears (for less than 30days) I have been charged R7 interest. That is an interest rate of 70%. A bit outrageous don't you think?