Product: Credit card

I am writing in regards to my CITI Best Buy credit card. I purchased a television for the price of {$1800.00} from a Best Buy store during a promotional period that offered 24 months of 0 % interest. I have been making regular automated payments that are well over the minimum payment required since the time of the purchase. I opted out of receiving paper statements sometime shortly after the purchase, but continued to receive various monthly emails from Best Buy, including statements, payment due date reminders, and confirmation emails for each automated payment that was received, as expected. However, the monthly email statements suddenly stopped in XXXX XXXX. Since my payments were coming out automatically each month, it did not occur to me that I was not receiving statements as I had for the previous year. I knew that my balance was very close to being paid in full. 
On Saturday, XXXX XXXX, XXXX, I finally received another emailed statement. Prior to this, I had not received XXXX since XXXX XXXX, XXXX - a full calendar year prior. On the XXXX XXXX, XXXX statement, the email noted that my account balance was now over {$1000.00}. I thought this must be a mistake, or that my account was used fraudulently. 
I contacted Best Buy credit card customer service first thing on Monday morning, XXXX XXXX, XXXX to investigate the issue. The first individual that I spoke to informed me that the promotional period with 0 % interest had expired on XXXX/XXXX/XXXX, and that I had now been charged an interest fee on the entire {$1800.00} purchase. I explained that I had received no notification, no statement since XXXX XXXX, and had no knowledge that this interest was going to be charged, or I would have paid the {$340.00} balance in full first. She informed me that if I had contacted customer service within 30 days of the interest fee being posted - by XXXX XXXX, XXXX - they could have reversed the charge. However, now that it was XXXX XXXX, XXXX - 6 days later - there was nothing she could do. I then asked to speak to a supervisor. I then spoke to a man who I was told was in a supervisory role. He was rude and unhelpful. He told me that the issue was my fault, and that I had not received statements because I selected the paperless option for my account. He provided no explanation as to why I received statements through XXXX XXXX, XXXX and nothing since then, even though I had not changed any preferences in my account from the time it was set up. I asked to speak to another supervisor. After attempting to refuse to transfer me, he finally agreed and put me on hold. 
I then spoke to a woman, who was also rude and argumentative on the phone. I explained the issue to her again. According to her, the system on her end showed that I was sent an email statement every single month since my account was created, and she could not explain why I did n't receive them on my end after XXXX XXXX, XXXX. I explained to her that this was in direct conflict with the information I was given by the previous representative, who told me I did not receive any statements because I opted not to. She offered absolutely no explanation for the conflicting information coming from her department. I explained that I felt I should not be held responsible for nearly {$800.00} of interest charged to my account, as I was not properly notified of the charge. I was also not given fair opportunity to contact customer service within the 30 day window that would have allowed for the charge to be reversed since I did not even receive a statement within 30 days. She refused to help me, and told me she would escalate my issue and I would receive a call within 48 hours. I never received the call, and called back 3 days later. I was told the previous person never even requested the escalation of my issue, and she had lied. The customer service dept now wants me to contact corporate to ask for a resolution.