Ticket change hidden/unaccounted for charges

Good day

This serves as an enquiry for a financially crippling service I received today when I changed my departure date.

I purchased a ticket last week on the 25 Oct 2018 for the following details: departure date = 30 Nov 2018 from Cape Town to Queenstown for R342 (student discount)

I later realised that I had mixed up my dates and went to the Intercape office in Cape Town Station today (30 Oct 2018) to ask for a change of tickets.

According to my understanding, a 10% penalty is applicable if the ticket is Full Flexi, which in this case was supposed to be R34,20, leaving an outstanding balance of R307,80.

Then a new ticket worth R323 (student fare) was detected on the search drive for the 23 Nov 2018, for the same departing and destination point. I believe I was supposed to pay the difference of R323 - R307,80 = R15,20.

But to my shock, I was requested to pay R80, which I enquired about and the cashier gave me an "explanation" behind the charge. I had to rush back to another commitment and I therefore had no time to take this up further with her as she seemed to be confident of her supposedly correct info.

Please assist me in this regard and explain to me why I had to pay that uncalled for amount. Additionally, if you acknowledge this error, I would like to request a refund as that would mean I have been ****** and I am merely a student that survives on the financial mercy of a single parent, thus every cent counts to me.