Over charge payment that can't be declared for after full upfront payment was made in advance

I have made upfront full payment regarding laser hair removal treatments which was a couple of thousands last year 2019.due to covid the beauty industry was shut down until the country was declared to level 3, then only I could make a follow up appointment. And on this specific day after I got my treatment the beautician told that I have to pay an extra R550, which I couldn't understand and was not also explained properly. Or given heads up to the reason or excuse of extra payment. I didn't question this verbally due to the fact that we were in receiption and other patients and clients were there. So didn't want to make a scene... I sent a personal message to the beautician asking her to explain this and declare it or provide evidancial reasons from her superior cause I understand that she only the liason. She said that she would take it up with them and get back to me and everytime I kept on asking she just kept on telling me that she would get bck to me regarding feedback the day was busy and couldn't find the files.. But she hasn't forgotten... And when days went passed and I query again her response was that she doesn't work there anymore. This laser clinic is based in Kimberley @ledmed hospital on the second floor. And it is apparently own by two doctors renting the space. I have spoken to the new girl all she could tell me is that unfortunately we can't do anything about that cause they don't know why I had pay again the previous time with my last session. How do I go about this cause I asked to speak to management was told they would get back to me. Up to this day the matter is till unresolved