Credit Balance Refund

Hi. I cancelled my Contract on the 3 June 2020. I had a credit balance of R654.60. I Paid my account a year in advance so I dont have the debit order running. So now in order to get my own money back i have to provide proof of payment which I don't have as the payment was made a year ago. I don't see the need to send proof that i paid the account in advance if whoever checked my account correct or if they checked it at all they would have picked up that it was paid in advance. Bank Statements does not go that far out. I have been struggling with this issue for over 3 weeks now and I am not happy with the service hence I cancelled my contract