Product: Credit card or prepaid card

I had a six-month zero interest on purchases promotion through my Citi Diamond Preferred Master Card. I also had a XXXX XXXX program balance through this account. On XX/XX/2021, I went online to pay my monthly payment, and the site presented several options for amounts. I selected to pay {$1400.00} because the site indicated that would be applied as follows ( quoting precise text/copy from the website here ) Remaining Adjusted New Balance + Flex Plan Monthly Payment This is the remaining amount owed to satisfy your monthly Flex Plan Payment Amount and the non-Flex Plan portion of your last statement balance. It does not include XXXX XXXX payments due in future months. 

This indicated to me that I would be paying off the purchase promotional balance plus my regular Flex Plan required payment ( normally {$450.00} ). 

Additionally, the printed statement states : Minimum Interest Charge. If we charge interest, it will be at least {$0.00}. 
How to Avoid Paying Interest on Purchases. Your due date is at least 23 days after the close of each billing cycle. We will not charge you interest on purchases if you pay your monthly Citi Flex Plan Payment Amount ( if any ) plus your New Balance, excluding any Citi Flex Plan balances, by the due date each month.

If you do not pay your monthly Citi Flex Plan Payment Amount plus your New Balance, excluding any Citi Flex Plan balances, by the due date each month, you will pay interest on your purchases from the date theyre posted to your account until you pay such amount in full for two billing periods in a row. We will begin charging interest on cash advances and balance transfers on the transaction date. We will begin charging interest on a Citi Flex Plan balance at the start of the billing cycle following the billing cycle during which you created the Citi Flex Plan. 

Again, I selected to pay the {$1400.00} amount on XX/XX/XXXX because the website indicated I would satisfy your monthly Flex Plan Payment Amount and the non-Flex Plan portion of my last statement balance. I knew that I needed to pay off purchases before the zero percent interest promotion ended, and that's what I was left with the impression I had done by making such a large payment. I later in the month made an additional payment of {$850.00}. 

On XX/XX/2021, Citi moved {$920.00} from " Promotional Purchase '' to " Standard Purchase. '' So, they are now attempting to collect standard interest on the {$920.00} and didn't pay it off as they indicated they would when I paid {$1400.00} on XX/XX/XXXX. I called today to dispute the way the payment broke down and was applied, speaking to two separate customer service reps, neither of whom would help me adjust the way the previous payment was applied. One of the reps told me that I needed to submit a screenshot of the text that their website showed me on XX/XX/XXXX indicating a payment of {$1400.00} would satisfy your monthly Flex Plan Payment Amount and the non-Flex Plan portion of your last statement balance. It does not include Flex Plan payments due in future months. I guess I'll hop in my time machine and go back to get that. 

The printed statement that I received at the end of XXXX ( attached ) DOES indicate that my purchase balance was XXXX. So at some point they DID apply the payment breakdown as they indicated they would and showed that I did in fact pay off the purchases balance. But on XX/XX/2021 they went into my Flex Loan account and moved {$920.00} into a purchase balance so they could start to collect regular purchase interest on it. Unbelievable.