Product: Credit card or prepaid card

I am the primary card holder of a CapitalOne Quicksilver Mastercard. In XX/XX/XXXX, I saw a balance transfer offer in the CapitalOne My Account section for my Quicksilver Mastercard. It was an 18-month balance transfer offer, with a 0% APR and 3% establishment fee. I requested a “No Hassle Check” in the mail to receive the $XXXX in funds. The letter included beside the check reaffirmed the terms: 12 months 0% APR, and a 3% fee. I cashed the check and was charged a $XXXX fee (3%) on XX/XX/XXXX.I hoped to see the terms of the balance transfer on my next statement, but they did not appear there. However, a 0% offer was clearly applied, as I was not charged interest on my $XXXX balance for the next few months.However: on my statement issued in XX/XX/XXXX, I was surprised to see a large interest charge for $XXXX, and my $XXXX balance apparently now subject to a 19.74% APR. By the time I could pay off the full balance on XX/XX/XXXX, I had accrued these interest charges, charged after the transition from the 0% APR to the 19.74% APR:XX/XX/XXXX	$XXXXXX/XX/XXXX	$XXXXXX/XX/XXXX	$XXXXXX/XX/XXXX	$XXXXXX/XX/XXXX	$XXXXXX/XX/XXXX	$XXXXA total of $XXXX in interest charges.In XXXX of last year, I attempted to get in contact with CapitalOne, first at a branch and then via mail, noting that the early expiration of the 0% APR offer was an error, and that CapitalOne had breached the terms of the balance transfer offer and charged me interest by mistake.However, I received a confusing response on XX/XX/XXXX: “the only offer regarding a balance transfer was not by check and the interest rate was the same as the purchase rate. This offer also included a 3% balance transfer fee”.This is incorrect. I did not use a random check that came in the mail with a standard purchase rate APR offer. I vividly remember going online, selecting the check, waiting for the check and cashing it at a particular bank branch, as I wanted to ensure I paid all my credit card balances in full so as not to accrue interest.Beyond my own memory, the letter suggests that I should have been charged “purchase rate” interest from XX/XX/XXXX onwards — which I wasn’t. The letter would suggest that it was a bank error in my favour — and not the reality, which is that my 12 month 0% offer was erroneously ended after approximately six months of a 0% APR period.I am still seeking a correction of this early expiration of my balance transfer offer. To resolve the situation, I request that my $XXXX in interest charges be refunded to me, either as a card account credit or a check issued to me. Before submitting this complaint, I called customer service in an attempt to resolve the situation and respond to the letter. The representative did her best with the information I provided, but could not find the terms of the balance transfer offer I agreed to in XX/XX/XXXX, stating that the system did not store information more than a few months old. She was able to uncover the “offer code” I used, which may have some value in CapitalOne’s further research: XXXX.After the lack of a useful response via phone, I decided to communicate in writing. I submitted the above request in a written letter to CapitalOne Customer Service XXXX XXXX XXXX in XXXX XXXX XXXX first on XX/XX/XXXX, then the same letter again on XX/XX/XXXXusing certified mail when there was no response. I received a short letter on XX/XX/XXXX that my request was being investigated, but after months of no further correspondence, I sent a letter on XX/XX/XXXX asking for an estimated time they might reply. On XX/XX/XXXX, I received a letter claiming "we are unable to fulfill your request through written correspondence", and a request to call their customer support number.I am now submitting a CFPB complaint as I do not think they will respond to my request otherwise.