Misleading Pricing and Disappointing Customer Service at Vodafone

I'm a Vodafone customer. In May, I paid 200 TL for a one-time 50GB internet package. When I checked my bill, I discovered that I had been charged 300 TL for the package. When I contacted customer service, they created a complaint ticket and assured me that a 100 TL credit would be applied to my next month's bill. After not seeing any reduction in my bill, I called again today to express my dissatisfaction. They initially claimed that the discrepancy was caused by a price increase. I explained that I bought the package for 200 TL, even though it was listed as 300 TL, and that the package price was actually less than 300 TL. Furthermore, because it was a one-time package, the price I paid should have been valid, and subsequent increases should not have been applied to me. They then stated that it was a system issue and that they couldn't help. There was no resolution, and I was basically told to accept the situation and remain silent. Because this deception is unacceptable, I have decided to change operators.