Product: Credit card or prepaid card

I activated a 18 month special financing offer ( no interest if paid in full within 18 months ) on purchases of {$540.00} or more that was offered by Citibank when I used my Sears Mastercard for the purchase by XX/XX/XXXX. I made a purchase on XX/XX/XXXX for {$550.00} using the Sears Mastercard. When I looked at my credit card transactions online, I noticed that the purchase was separated into two transactions ( {$540.00} and {$15.00} ) so the special financing wasn't applied. I contacted the credit card company on XX/XX/XXXX and was originally told that there was nothing that could be done. I asked to speak to a supervisor and she told me that this situation happens frequently and she gave me an example of how airline companies separate transactions into the individual ticket amounts if more than one ticket is purchased in a transaction. She said that they combine the transaction amounts in order for the promotional financing to apply. She said there would be no problem with my purchase being applied to the promotional financing. I received a letter from the credit card company dated the following day ( XX/XX/XXXX ) that said they received a request regarding the promotional terms for the purchase I made on XX/XX/XXXX and that I would be notified within the next 2 billing cycles of any changes that are made. I notified the credit card company by phone again about my concerns with the interest that I was being charged while the request was being reviewed and I was told not to worry because the monthly finance charges would be credited back to my account once the promotional financing was applied. I received my credit card statement on XX/XX/XXXX and the promotional financing was still not applied and I was still being charged interest on the purchase. I called the credit card company again and was told that I didn't meet the minimum amount required for the promotional financing. I had to explain the situation again and was put on hold for about 15 minutes while the customer service representative was consulting with a supervisor. I was told again that the minimum wasn't meet so nothing could be done. I asked to speak to the supervisor directly and was put on hold for close to 30 minutes waiting to be connected to her. She also said nothing could be done and it was already reviewed by the promotional financing department. I asked why I was never contacted about the decision to decline my request and she apologized for not being contacted. I explained to her that the supervisor I originally talked to said there would be no problem with the promotional financing to be applied. I asked for the name of the supervisor that I talked to and she said that wasn't listed in the notes. She said she only knew it was someone from her department and that I called at XXXX XXXX and that the notes from the phone call was entered at XXXX XXXX. I asked for an email address in order to file a complaint and she said there was not a way for me to file a complaint. I asked how I could contact the promotional financing department and she said there was not way to contact them either. I asked how supervisors send the requests to them and she said there is a way to do it through their system but there was no way to contact them outside of their system.