Telkom's excess charges

I decided to give Telkom the benefit of the doubt and pursued to open a Wifi contract with them. I opened my contract around the middle of March 2018 for a package of R269 per month and my excess data usage was set to 0 by the consultant. I was informed by the consultant that I would pay the pro-rata amount for March and the full amount for April at the end of April. That went smoothly until I, to my surprise and irritation, received a bill of R282.87 for May. Now I am unsure of whether this is a mere glitch in the system or if Telkom usually takes advantage of its clients this way, but if the latter is the case then I will definitely take this further. I am highly frustrated as R269 is what is stated in my contract. Telkom must rectify this matter.