Pathetic service and fraudulent dealing

I am forced to give 1 star they don't deserve it. I bought a computer box set for renault from them they charged me R2700. I drove for 30min to get it as I lived far from them. The guy told me that anything not working I can bring it back , so I went and fitted the stuff to my car and vehicle refused to start. Next day I went back to them the same guy says his boss said we can't refund but can give you a credit note as they don't have a replacement set. So I was forced to take a credit note. After a month I needed a computer box for peugeot so I went to them so I can use the credit note. Now the guy says we can't use the credit note as his boss said . This was daylight robbery they are selling stuff that doesn't work and defrauding people. Can some one assist me in this matter please.