Product: Credit card or prepaid card

On XX/XX/2018 I was charged a ridiculous amount of interest of {$65.00}. My balance was {$900.00} beforehand. My apr is 26.99 % of interest rate. The credit card has this thing called Promotional Balance where if you spend X amount, then it qualifies for X months of no interest as long as it's paid in full. Okay, I understand that. The problem arises when there's four promotional balances on my account. Where one of the purchases for {$340.00} was added to my account. The thing is that I refunded that purchase of a TV. I got the money back, but the promotional purchase was never removed on my account. I didn't think much of it since the refund should've covered it. Then, the promotional balance stopped showing up on the homepage of my online banking account. Didn't think much of it as nothing happened. Okay. From looking at my statements I see that normal payments were applied to the promotional balance until near expiration even though that wasn't supposed to be the case.
Anyways, the promotional balance of the following balance {$370.00} was set to expire on XX/XX/XXXX. The date passes and nothing happens until six days later where I was billed {$65.00} of interest because the balance was not paid in full. It didn't make sense so I called up customer service of the Best Buy Citi Bank and explained the situation. The lady there told me I only had two promotional balances, I never had three nor four of these balances according to her. Which didn't make any sense to me.

Then, she explained to me how they process these promotional balances is that the non-promotional balances must be paid first in order to add any of the payments onto the promotional balance itself. That is online. She then goes on to say if I would've called and specified that I want a payment to go to the promotional balance that they would. The thing is I never call to do my payments. I always do them online and no where in the fine print on their website when making a payment does it say that none of the extra money past the minimum payment does not go to the promotional balance. Then what is the point of making a payment anywhere, but on the phone?

Finally, this leads to where I was never told of this information and didn't know of it because it didn't explicitly say this on their site when making a payment. The only time I've called was to activate my credit card and that was it. Never mentioned anything about payments. So the lady then goes on to say if I pay the remaining ( about ) {$290.00} then she'll remove the {$65.00} of interest on my account. I go on to refute this statement with saying I've made multiple payments that go way past the minimum payment of sometimes being {$50.00} to {$100.00} or more a month. She says that was never applied to the promotional balance and goes on to repeat herself of paying the remaining amount she'll be glad to remove it which wasn't helping my scenario at all whatsoever. It's quite ridiculous that they do this and don't treat me as a valid customer in repeating their script over and over to the point that the only fix is to go through the operator on the phone and talk to a representative specifying that I want a portion of the payment towards the promotional balance when I knew nothing of this whole issue until it affected me.