DFS problem with interest charged on cash misapplication

Simple problem. I made a payment to Dell completely paying off my current balance and more than the required amount on my 6 months no interest charge. Dell paid off my 6 month no interest charge and left a current balance which generated a $17 interest amount.

I paid off the account again in full including the interest and contacted Dell Financial Services. The somewhat clueless rep wondered why I didn't want to pay interest but did eventually try to reallocate the money correctly.

Next statement showed my payment of the complete balance, the reallocation which should have refunded my $17 but somehow generated another $5 in interest. I have a nice email saying to me valued customer that the problem has been fixed.

I paid the $5 and wrote a letter with copies of the statements and mailed it to the given address for Financial Service correspondence. After not hearing anything for a month I tried to open a support request with Financial Services but I am back in support with robotic letters, confidential information needed again. Nobody documents anything or responds.

I would love to get my interest refunded and close my account and never hear from Dell again. I have already wasted more time than this is worth by far!

Can anyone help without me investing hours on the phone and potentially getting another bill for interest on an account that has been paid off twice?

Konrad U