Product: Credit card or prepaid card

I made a purchase on this card in the amount of {$520.00} on XX/XX/XXXX. I was to pay no interest on this purchase for 18 months as long as the balance was paid in full by XX/XX/XXXX. The total amount of deferred interest was {$250.00}. I have paid over the monthly payment amount each month since this purchase, yet only {$64.00} was applied to the promotional balance. From XX/XX/XXXX to XX/XX/XXXX, I paid {$460.00} alone on this card. I called the credit card company on XX/XX/XXXX when I noticed I still had {$460.00} left on my promotional balance despite paying well over that amount in the last 18 months. I was told it was because I had regular purchases that accrue interest daily so my payments were applied to that first. I requested to speak to a manager and I was transferred to " XXXX ''. I asked her to reapply the payments that I made since XX/XX/XXXX so they were applied to the promotional balance instead of my regular purchases and she refused to do so stating my promotion was ending that day. It had not ended and I was still in the promotional period but she just refused to help me. My regular purchase balance was never {$0.00} for the 18 month promotional period. Their practices are deceptive applying little to no amount of my payment to my promotional balance each month leaving me with a balloon payment and then REFUSING to help me when I called to resolve the issue. This is unfair practice. 

From XX/XX/XXXX to XX/XX/XXXX, I made {$1400.00} in payments and only {$64.00} was applied to this promotional balance and that was part of my payment in XX/XX/XXXX. I did have a promotional balance for {$320.00} where {$34.00} was applied to the month before the promotional balance ended and then I had to make a lump sum payment of {$280.00} to pay that off in XX/XX/XXXX. I paid almost DOUBLE both of my promotional balances and only {$98.00} was applied to them the month before they expired then I had a balloon payment due the month the promotion expired. Why is Best Buy inconsistent and applying only a PARTIAL amount of the payment to the promotional balance and only doing so the month before the promotion ends? What kind of deceptive and unfair practice is that? 

There is NO OPTION online to confirm how you want your payment applied. I was told that I would have to make the payment, wait for it to post, and then call them every month and have them reapply my payment. This is absurd not to mention I do not trust them to not mess up the accounting of my card when they reapply a payment each month. I am the consumer and the one making the payment. I should be deciding how it should be applied in MY best interest, not the interest of the company. There should be an option at the time you are making the payment where you can decide how you want it allocated. Many other companies do this and may other companies I use for various reasons. But why would they implement this when they are making more money off consumers the other way? They act like you have a period of time to pay off a promotional balance but they misallocate your payments so you have to pay one lump sum at the end anyway despite the payments you made all along. 

At the VERY LEAST, any amount over the accrued interest for that month should be automatically be applied to the promotional balance since that has an end date and regular purchases do not.