Product: Credit card or prepaid card

The charges that the company are alleging have occurred are on a closed account for a canceled service ( I-the customer have canceled these ). Best Buy did go back and credit part of the balance, but they are not taking care of the late fees associated with wrongly charging this card. 
The Sales Associated recommended applying for and using a Best Buy card when we checked out at Best Buy XXXX on XX/XX/XXXX. He suggested using it for the great discount and said that we could pay it off when we were at the register before leaving the store. This is what we did do and these receipts are available if needed, but arent in question. The representative also recommended Best Buy XXXX XXXX service to help with a computer. We were unfamiliar with this service, the associate explained how it worked. He said that we would really like the service since we live an hour and fifteen minutes from the nearest store and we can drop off the item to be repaired or we can call and get assistance over the phone. 
On XX/XX/XXXX, I brought a tablet computer to the store for repair. Upon getting to the XXXX XXXX XXXX, I was met with rude service. This repair service man was quite angry that I brought a tablet to be looked at without a previously scheduled appointment. He refused to help me and was speaking to me in such a way that a higher up in the department came over to help. The higher up agreed to take the tablet and that they could check it in when they had time available. 
When I returned to pick up the tablet the service was not much improved, with the XXXX XXXX member telling me he was too busy to tell me what problems had been repaired. 
With these being my only interactions with the XXXX XXXX, I immediately ( while at the checkout desk ) asked for the tech support service to be canceled. Proof of the cancellation is included and is dated XX/XX/XXXX. 
Since then, I have not been in a Best Buy store or utilized their services in any form or fashion. 
In XXXX of XXXX, I received a letter from CitiBank that my account was delinquent. I didnt have a CitiBank Card XXXX so I called and asked what this was referencing. When I called the lady asked for my card number, however I didnt have a card number. She could tell me that it was in reference to my Best Buy card. To clarify, I have never received a Best Buy card in the mail and have only used the store on XX/XX/XXXX, XX/XX/XXXX, and XX/XX/XXXX, and always used my social security number on the keypad and paid the total before leaving the register. This CitiBank associate couldnt help me and advised me to call the local Best Buy store. 
I did call the local store the same day, but they couldnt help me since it was a credit card issue. 
It wasnt until XXXX of XXXX that I received a Best Buy Account Statement that showed an open balance, but wasnt itemized. When I called them, the associate gave me three different totals that were due ( the totals all varied between XXXX and XXXX ). She said the charges were from renewing my XXXX XXXX membership. I did explain that the membership was canceled well over a year ago and that we hadnt returned to the store or renewed the membership in any way. She said they would need to look into that, Id be receiving a letter in the mail in 6-8 weeks with their decision. They did deny my claim and refused to credit the account. 
I have received monthly statements since this time and have called and asked about the charges nearly monthly. The charges have changed, but none of the associates can give me further information about them. I have asked to be forwarded to a manager, but have been unsuccessful in speaking to them. 
In XXXX of XXXX, I received a credit collection letter from XXXX XXXX XXXX. Upon calling to discuss the previous events with them, she said that they would reach out to the company and look into the charges. They are stating that CitiBank reviewed the account and says the balance is valid. I greatly disagree with this for the above reasons and have evidence to support this being an unauthorized charge.