Product: Credit card

On XX/XX/2016 I purchased a hot water heater at the local Home Depot. I used my Home Depot Credit card for this purchase. This purchase was a " no interest for 6 months payment ''. The total amount for this purchase was {$530.00}. The promotion expiration date was XX/XX/2016. On XX/XX/2016 I paid by check a total of {$530.00} for the water heater, this is the amount noted on the account as the full purchase price. On that day I also paid, by check {$50.00}, for the payment due on the Revolving Balance of my credit card account. I paid the full amount for the water heater at that time because I had received my IRS tax refund. The check for this payment cleared my bank checking account on XX/XX/2016. 
I unfortunately did not notice, in the following three months ( XX/XX/XXXX, XX/XX/XXXX, XX/XX/XXXX ) billing cycles that the " no interest for 6 months promotion '' was still listed on the account bill as not being paid. 
On XXXX XXXX, 2016, I went to the Home Depot where I purchased the water heater, and spoke with a customer service employee about the continuing listing of the water heater purchase on my monthly bill. I have the original cancelled check for the payment made on XX/XX/2016. The customer service employee was able to verify that the payment had been made, and provided me with a copy of the transaction. She advised me to contact Home Depot Customer Service at their XXXX number. 
Today, I spoke with XXXX at Home Depot Customer Services. I explained why I was questioning the continued billing for the water heater when I have the proof that I paid for this purchase XX/XX/2016. She explained that the check for {$530.00} was applied to general revolving account, not to the specific Promotional account. She further explained that IF I had submitted the total payment for the water heater on or near the expiration date of the purchase, the amount of {$530.00} would have been applied to the Promotional purchase. If I had wanted to have the Promotional purchase amount paid off early, I should have called Customer Service at Home Depot to alert them of my intention to pay off the purchase earlier than the expiration date. Since the expiration date for this promotion has expired, she explained that the only way I can have the purchase removed from my bill, is if I pay ALL interest due for ALL purchases for 2016. The amount I have to pay to accomplish this is {$500.00}!!!!. In other words, I am being penalized for paying the promotional purchase early!!!! 
I asked why the Promotional Purchases are not billed separately to avoid the unknown penalty. Her answer was that is the way the computer is set up. The clerk at Home Depot when I paid the promotion purchase, never told me that I needed to call Customer Service to alert them of my intention. 
I do not have {$500.00} to pay them to have this item removed from my bill. So it appears this item will continue to accumulate interest forever. 
I feel I and ( others I am sure ) are being treated unfairly by Home Depot as this is an unfair practice on the part of this company. 
My desired resolution is to have this item removed from my account and to be noted that it was paid in full, with no additional interest being charged.