Not working according to their own policy

I purchased a gasket set from goldwagen but once getting it home I was not happy with the quality so decided to return it the next day, unopened, not damaged and in it's original packaging with the invoice. When I got there I was told I would be getting charged a 20% handling fee. According to their return policy it says" unwanted, unused parts will incur a 20 % handling fee if returned later than 5 working days of invoice date". I returned my goods the very next day so why was I charged. When I questioned the owner he simply said that's what our policy says and refused to listen. Other customers had returned goods while I was waiting and not one of them was charged the 20%. I will be taking a copy of the policy to my lawyers in the morning to get clarity on my rights.