Product: Credit card or prepaid card

XX/XX/XXXX : Signed up for Citi Diamond Preferred credit card with 0 % intro APR on balance transfers for 21 months. As part of the online signup process, I did a balance transfer in the amount of {$5100.00} from another credit card, which was subject to a 5 % transfer fee on the {$5100.00}, calculating out to {$250.00} transfer fee. 
1st Billing Period - XXXX : Bill arrives in mail with a total initial balance of {$5300.00} ( {$5100.00} + {$250.00} transfer fee ). My plan is to pay the credit card in full before expiration of 0 % 21 month promotion period. I made my 1st payment in the amount of {$250.00}. I get this monthly payment amount by taking $ XXXX {$250.00} and rounding down little. 

2nd billing Period - XXXX : 2nd Bill arrives with new balance total of {$5100.00} ( {$5300.00} - {$250.00} from first payment ). I repeated the $ XXXXmo. payment all the way through Billing Period XXXX which had a due date of XX/XX/XXXX. This took my 21 Mo. 0 % APR credit card promotion balance down to {$3100.00}, with approximately 12 months remaining on the promotion period. 

Citi Flex Loan Offer : On Monday, XX/XX/XXXX I applied for a Citi XXXX XXXX in the amount of {$11000.00} to pay for a small home remodel. I was able to do this because Citi gave me an initial credit limit of {$14000.00} when I signed up on XX/XX/XXXX and I currently only had a credit card balance of {$3100.00} as of XX/XX/XXXX. The Citi Flex loan request was approved on Thursday, XX/XX/XXXX for the amount requested. The FULLY AMORTIZING LOAN TERMS are : Loan Amount - {$11000.00}, Plan APR - 5.99 %, Plan # payments - 60 months, with a FULLY AMORTIZING MONTHLY FIXED LOAN PAYMENT AMOUNT of {$220.00}, which is shown on the documentation I received from Citi. This is very important to remember and will be explained why below. 

Billing Period XXXX : Credit Card bill shows up in mail with the new Citi Flex Loan amount of {$11000.00} included on bill, along with existing 0 % APR Promotion Balance amount of {$3100.00}. New total credit card balance is now {$14000.00} ( {$11000.00} + {$3100.00} ). 

Moving forward, the plan was to make a monthly credit card payment in the amount of {$470.00} to Citi. This new payment amount is made up of $ XXXXMo. I have been paying on the 0 % APR Promotion Balance & now the new Citi Flex Loan fixed monthly payment amount of {$220.00} provided to me by Citi. This will allow me to pay off the 0 % APR Balance before promotion period expires on XX/XX/XXXX and then just keep making the $ XXXX/Mo. payment toward the Citi Flex Loan balance after that. 

Instead of the above happening, Citi is applying the entire {$470.00} monthly payment amount, minus the minimum payment amount due on the 0 % APR promotion balance portion of the monthly bill, to the Citi Flex Loan balance. IN ADDITION TO THIS, Citi is charging me additional monthly interest on the outstanding Flex Loan balance, even though the fixed monthly payment of {$220.00} is calculated as a FULLY AMORTIZING LOAN WITH A 5.99 % APR. This is how Citi calculated the monthly payment to be $ XXXXmo. Every month Citi adds an additional line item on my bill with additional interest to what is already calculated in the XXXX XXXX monthly payment. HOW CAN CITI CHARGE ME EVEN MORE INTEREST ONTOP OF THE AGREED TO FIXED APR INTEREST RATE OF 5.99 % ON A FULLY AMORTIZING LOAN?!?!?! 

I did not pick up on this error until early XXXX. I called and spoke with a Citi representative on XX/XX/XXXX and explained this clearly to him. He agreed with me, put me on hold and eventually got back on the line and informed me that he submitted a request to have this corrected. Instead of getting my next bill in the mail with the errors corrected, I got a letter from Citi that basically said pound sand and we can apply your monthly payment the way we best see fit. I have now received the next bill in the billing cycle and it has not been corrected. Citi is showing my 0 % APR promotion balance amount as {$2700.00}, when it should be {$1300.00} based on all the payments dating back to first bill.. 

Think of it this way, I have to pay off my current balance on the Citi Flex Loan of {$8700.00} with a 60 month payment term, before I can even attempt to pay off the 0 % APR promotion balance, which expires in XXXX XXXX. So now I have to wait for the higher interest rate APR to kick in on the expiration of the promotion period and most likely have to pay the deferred accrued interest charges from the promotion period, before I can pay this portion of my monthly bill off?!?!?! I am filing a formal complaint with your office to assist in this matter, as anyone I speak to via the Citi customer support line will not be able to fully process the gravity of this situation, nor will they have the authority to resolve the issue in a timely manner.