Product: Credit card or prepaid card

I received and activated a 0 % APR offer from Citi in XX/XX/XXXX which would run through XX/XX/XXXX. I ended with a final balance of {$7400.00} as of the end of the promo period with no new charges after the promo ended, and I had more than the amount available in my checking account to pay it off in full. I asked customer service in XX/XX/XXXX when I needed to pay off the balance in order to avoid accruing interest as I know different banks calculate interest differently and am not always able to discern from their disclosures how it will work ( e.g., Citi 's disclosures on statements say both that interest is calculated daily and that if the statement balance is paid in full on the payment due date then no interest will accrue ). I've often seen that the 0 % interest will remain on purchases up to the promo end date so long as the balance is paid in full by the payment due date for the billing cycle in which the promo ended, which is what I thought I understood from the disclosures and what I was contacting customer service to confirm. In this case, the billing cycle during which the 0 % promo ended closed on XX/XX/XXXX, and my payment due date was scheduled for XXXX. The rep said that I should rest assured that as long as I paid off the statement balance on the payment due date, I would not accrue any interest, so I scheduled an automatic payment for the statement balance on the card to be paid XXXX. While reviewing my recent transactions today, I saw that I was charged {$30.00} in interest on the previous statement 's close date ( XX/XX/XXXX ) for the days between the ending of the promo ( XXXX ) and the close date. I contacted customer service today, XXXX, and they explained that Citi calculates interest daily, so I started accruing interest the day after my 0 % promo. I explained what I was previously told and requested a waiver/refund of the interest as I felt this was a case of promissory estoppel, i.e., I scheduled my payment for the full statement balance on the payment date rather than the promo end date on good faith that customer service was providing me accurate information. I further paid off the entire balance ( {$7400.00} - slightly lower than the balance as of a month prior due to the previous month 's payment and some return refunds ) today, XXXX. The waiver was denied, and though customer service claims to have escalated the concern, they also told me I will accrue ~ {$110.00} in additional interest on the current cycle that I will have to pay off and further dispute until I have made two full payments to Citi. I told them it seemed ridiculous that I would continue accruing interest and having to pay for acting on the wrong information from customer service, and while the agent was sympathetic, she said there was nothing more she could do. She furthermore could not provide me with a timeline of when I might be hear more about the escalated complaint and/or its resolution. Given that Citi has previously charged me interest in situations where I was told by customer service that I would not be charged interest, and given that they haven't always refunded that interest when they said they were going to, I'm filing my complaint ahead of waiting for further contact from Citi.