How do I contact someone that can actually fix my billing issues?

I added a line to my cell service and purchased a phone at the xfinity store. The phone was supposed to be 600 dollars. They ended up charging me a 1000 for it and saying I wasn't eligible for this price after I agreeing to it. Ive spoken with 4 different people trying to resolve this. The first said they would apply a 30 dollar a month credit to my bill to offset the charge. The second one said they didn't give me the phone for 600 because my plan wasn't eligible, but if I upgraded my plan they would discount the phone. Obviously i didn't want to do this but I agreed. He also said he would put in a ticket with their promotions department to get that plan reduced to my old plans price.  No discounts, but my bill went up immediately. Thirds person just agreed with everything and said it would take 10 business days for the discounts and multiple credits to show on my bill. Waited over a month, so I figured I could speak to the people that sold me the phone. The manager told me I should have read the fine print and it was my fault. Ok if that's the case then why did all of the other people I spoke with agree with me offering bill credits from each of them? Then tell me if I upgrade to premium plus they would fix it? Nope they just suckered me out of more money. The store manager did note that he could see the notes on file where I have contacted them several times and all he could do was submit another ticket. I need to speak with someone with some actual authority to fix this issue or ill take my business somewhere else. Ive been a customer with xfinity for years and this feels like a slap in the face.