Ridiculous "interest"

My mom had a R2400 credit then took a fridge and made an upfront payment of R2500 as the fridge cost more than the credit she has, and that was last year June. We've been paying R500 monthly and only skipped one payment but now it's said her account is outstanding R6000. We went to query about that in store and the head office was contacted and they told us that it's because of the 1 month that was skipped and when we asked for a detailed statement to be emailed, they failed. How exactly is the business run here because there is no way 1 skipped payment can cost this much? This is ******* nje