Unauthorized debits

I bought furniture from Ol Furnishers and last Mont was supposed to be my last payment. The last instalment was supposed to R368.48 as per their monthly statement however they debited more than that amount. I reversed the debit with my bank only to receive a call from Rainbow that instalment has now jumped from 377 to 467. How,why and calling their call center is useless because the calls are never answered. I emailed OK but also received no response. During the contract period I had to deal with unauthorized double debits which at times left me with charges as I had not made provisions for the deduction. Somebody please explain what's happening at this company. I think now that I have settled the account they see it as an opportunity to repossess the goods and play games with customers.