Product: Credit card

I have been having an issue with JcPenney 's credit card service. I happen to work part time for the company & for me to get my full associate discount, I am told I need to use the card or cash. So like every-other financial product I use I track every payment on a excel spread sheet. Around XX/XX/XXXX I noticed that when I would make a payment online they were not applying it to my balance. I had a balance of around {$700.00} I went online on XXXX/XXXX/2015 & paid {$320.00} ... which it shows on the website. Then ten days later I went on again to pay it off ... .the {$320.00} payment had not been deducted ... there should have been a payment of {$290.00} remaining to pay it off. I made that payment online on XXXX/XXXX/2015 ... ten days after the first one & it was not deducted. Then to make matter worse they were either slowly deducting & not fully deducting payments so my balance no matter how much I pay is always just below {$800.00} higher than it should be & they are charging me 25 % interest on that balance. I have tried to call them, contacted them emailed them numerous times & they deny it ... ..I started making my payments in the store. Until this last email they admitted it, but are asking me to prove it. It is listed on their website as a payment I made ... .I have screen shots of my payments ... yet I have to prove it? The thing that kills me was when I called in XX/XX/XXXX they told me after getting very upset with them that my balance was really {$180.00} ... not the {$1000.00} something they were showing on my statement. I said then why is n't it showing that? So they must have a record of the payments I have made even though they are not showing me this in the final statement ... this has been going on so long & I have been charged a huge amount of interest during this. They keep telling me that it is all correct because I am spending but I spend then pay ... because that is how I get my associate discount. But here is the thing I have the right to pay & decide how much of their interest I want to pay by making big payments ... except they are n't being honest. Here is what they finally replied to me ... but if they know they did n't apply it why do I have to prove I made these payments? 
" Our records show that the payment in question has not been credited to your account referenced above. To assist us in resolving this matter, we need documentation of the payment. Please review the list below and, depending upon how the payment was originally submitted to us, please send us the document ( s ) needed to complete our research. 
- Mailed Payment Processed Electronically ( Only if the copy of the check is not available from your bank ) : Please send a copy of your bank statement or a letter from your bank stating the check cleared, the date the check cleared, whom it was made payable to, and the amount of the payment. 
- Mailed Payment : A clear copy of the front & back of your cashed check or money order. Please note that the receipt stub or carbon copy for the money order will not assist in our research. 
- In-Store Payment : A copy of your receipt. If a check or money order was used, a clear copy of the front and back of the cashed check or money order. 
- Online/Electronic or Phone Payment : From the online provider, a copy of the transmittal report for the payment. Please note that a copy of the scheduled payment or a bank statement showing the online payment being withdrawn will not assist in our research. 
Please mail this information to the following address and include a copy of this email : JC Penny card XXXX XXXX XXXX XXXX, FL XXXX or Fax : XXXX Upon receipt of the above outlined information, we will take the necessary steps to see the proper adjustment ( s ) is made to your account. If we do not receive your response within 20 days of the date of this letter, we will consider the balance to be correct. '' So what legal action can be taken?