How can they promise one price for a year than change it 9 months later?

How can they GUARANTEE a fixed rate then just up my bill $6.85 for no reason?

I called during COVID in March 2020 for "help" to lower my bill. The customer service person said they would help and they lowered it $20/ month. Ten minutes after the call my cable TV was turned off. I called back. The new customer service person said they no longer offered the package I had, They returned my basic cable Tv minus Starz for $10 more per month than I had been paying.They said the plan was good for one year. Now I have automated payments and I just noticed that my bill went up $7,38 in December. Why? I have an extra charge of $6.85 and with taxes it is an extra $7.38. Why? I hate Comcast!