Product: Credit card

XX/XX/year> ( XXXX EST ) - I received my XX/XX/year> statement for my credit card and saw that I was charged interest for the previous month. I pay my charges in full every month, so this was strange. 
I called customer support and they helped me see that the issue began with my XX/XX/year> statement. 
I started a Flex Plan In XX/XX/year> for a large charge. Per the Flex Plan agreement, I incur monthly fees for that plan and pay a lump sum over 6 months. 
I made a payment on XX/XX/year> that covered all of my charges except for the Flex Plan charge. Citibank misapplied that payment and applied {$89.00} to the Flex Plan, reducing my Flex Plan balance and increasing my current due balance. I did not expect this and it caused me to incur interest on my regular charges from XX/XX/year>, as the balance due was higher than it should have been. 
My interest charges were reversed by an agent. 
The issue is that the misapplication cause me to have to pay more each month to cover the current purchases in order not to incur interest. That, and if the payment is being made early on the agreed upon Flex Pay schedule, then that is breaking the agreement and I want a reduction of my fees for the Flex Plan. 
I asked for the math behind the {$89.00} applied to the Flex Plan and why it was applied before the agreed upon payment was due. I was read payment application policy information, which does not agree to how my payment was applied. I spoke to 3 representatives about this for almost 4 hours until I got to a supervisor ( XXXX XXXX XXXX XXXX ) who submitted a formal request to Citibank 's " back office ''. 

XX/XX/year> - I received a letter in the mail dated XX/XX/year> from Citibank referencing the payment policy again and saying they would not adjust my payment application. There is no further explanation or concern about the exact problem I am facing mentioned in the letter. 

XX/XX/year> ( XXXX EST ) - I spoke to an agent ( XXXX XXXX I only have good things to say about him ) who worked through my questions and the math with me and he understood my complaint. However, he did not have authority to correct this. 
He escalated me to a manager XXXX XXXX- though I did not hear his name clearly ), who did not listen or look at the details of the problem. He merely read the payment application policy to me over and over without doing any further work into the issue or listening to me explaining the details of the problem. He frustrated me to the point of yelling at him on the phone and I asked to be transferred. He refused to transfer me, saying I was already at a manager level and could not be transferred. 

XX/XX/year> ( XXXX EST ) - I called back and I have spoken with a second supervisor who understood the issue, and is trying to have this corrected. I also spoke with her on XX/XX/year> ; her name is XXXX XXXX XXXX XXXX. She has sent an email to the back office asking them to correct this payment. She assured me that this should be corrected because, a direct quote, " you are correct about your payment being misapplied ''. 

I have spent 6 hours of my personal time trying to have this looked into and corrected. I have gotten people that were nice, but had no authority to help, and those that could help, either will not, or have to forward this to another department. And then I have to start all over again, feeling like I'm being run in circles until they expect me to give up. 

I certainly will be closing my accounts with Citibank as soon as I clear up my amounts due. 

Thank you for your time looking into this.