Unauthorised charges by Uber South Africa

I have noticed over a period of 3 months, that Uber are debiting the sum of R50, for no service that I am aware of and which does not appear in my history. Trying to query this takes one in circles on their websites and once you have eventually filled in a complaint and you click on submit, nothing happens. It appears to me that once they have your credit card details they just debit it.