Incorrect balance on tfg account

I have an account with tfg foschini group They have me credit for R468 in September which required a deposit that I understand goes towards my instalment paid first instalment in Dec skipped Jan and paid end Feb only to see my balance was R850 from R400 the store can't explain what happened nor assist me they printed a mini statement with interest and collection fees for only a month skipped they charge for calls as well help me understand tfg how can I pay so much money