Why is my bill different every month?

I’m confused at how my bill changes every month. I’m a new customer, since February, and it hadn’t ever been the same, anywhere from $76.23, to $90.00! On top of that, they won’t take my ssi debit card, in which everyone else does, so I have to call it in. They are supposed to give me a 5.00 discount, but they are “accidently” adding it. I even tried to go down on my data, to save money, but nope, it’s still $85.00! I signed up under the impression that it was supposed to be $69.00! That is what I was told.