Interest rate

I brought a bed at Lewis the bed was R7999 I paid two in instalment went to pay to try push my account I asked for the statement since I wanted to finish the account by December I was not happy by what I got the statement it says I have a outstanding amount of R10000 something which didn’t make sense to me when I ask for explanation I was told that every month my get interest even if am paying which still doesn’t make sense how can I hv interest while I’m paying they further explained to me that in 24 months the total amount will be about R18000 something which is also wrong cos i brought a bed for R7999 nvr agreed to pay for 24 months I am not happy with Lewis .I brought the bed using my mom’s account since I’m not permanently employed.