Incorrect billing... AGAIN!!!!

I downgraded my account earlier this year from 120gig (R699 per month) to 10G (R99) as i believed i was paying far too much for data. Since the downgrade, i have been having issues with incorrect billing and this has been really frustrating and kinda defeats the purpose of the downgrade entirely. I have been getting billed R197 since the downgrade and this is grossly erroneous, and not only that, the billing date seems to have been changed by Telkom. I made it very clear that I prefer to get billed on the 25th but now I get billed randomly now. Last month I think i got billed on the 15th, which is really odd to me. This is unacceptable. I cannot be ****** like this by a service provider. If this is not corrected then i would like to cancel my account!!!