Terrible Service when you need them but when a payment is a day late you get billed and called every 5 minutes!

This is now my last resort as I do not know what to do any more. I have made a payment incorrectly - I paid my installment towards RCS Loans and into my husbands old loan account which had been paid up. I have informed RCS Store Card as soon as I was made aware and I have asked numerous times to please allocate the payment to my account as I am being held responsible for all the legal fees and communication being send out to me but I do not get any feedback from you. I have send numerous email as it is difficult to talk over the phone during the day as I am a debt collector myself however I have taken calls as well. I have requested numerous statements, I have send everything though, I have called in you name it I have done it. Previously RCS debited my account incorrectly and also charged me extra fees for non payment, my bank charged me fees as I had to dispute the debit order as RCS could not correct their own mistake. Still I had to pay those fees as well. Please can someone get back to me in order to allocate this payment and advise me the way forward regarding all the fees and costs being added for no reason as RCS Store Card is not responding.