Unfair payments on product already paid

I bought a dinner set. Paid it finish in 6mnths. They told me that I paid a shortage as what I paid there is a R160 difference. I had to paid that as well as the monthly interest and admin and paid another R627 which was a total of R4077. Yet they want another R404 now and I paid the product finish in 6mnths. What kind of business is this that expect you to pay interest and monthly charges whilst you paid it finish. Wow I'm soooo upset with this Homechoice they are dishonest. Plz be careful to rather not buy or pay ur acc finish soonest. Yoh. Why must I still pay??? With Covid having so many lost jobs and cannot pay yet they want to milk a person.