What is a Once-off fee - MADNESS

Telkom is such a cheat. They called me to guarantee me that they will be giving me a special and I should approve the debit order for a R599 subscription on my 30Mbps as of September 2025. My invoice stated for the month of August to pay for a subscription and once off fee of R245 extra on my account and this has been paid in the debit order for 31 August 2025. Now I get the same billing of subscription and once off fee for the month of September that must be paid again. How does this happen as far as I am concerned a once off fee is paid once and not Billed the following month. I think this is just taking advantage of me due to the fact that I was complacent with the first debit order and now again. I don't think Telkom understands what a once off fee implies.