Dont want to refund my money-Goods not working cor

We bought a intercom system (Scala Rider) from Full Throttle on 12/09/2015. They install it in the new helmets that we bought from them. On our first trip we realised that we could not hear each other when driving at a speed of 50 km/h. We took it back on 28/09/2015 and they replaced one speaker in the helmet. Still we could not communicate while driving. We went back to shop again and told them that the system is not working well. We decided to try a different intercom system. They then installed a Interphone system in our helmets. After testing it on the road we realized it worked better than the Scala Rider but at a speed of 90 km/h we could not communicate or hear each other. We once again returned to the shop and told them that this system also doesn't work to our satisfactory. They took the Interphone back. We asked them if they can get us a SENA intercom system because we know that that system works well.(Do own a set in a other helmet) They phone me on the 12/10/2015 and told me they can get it for me for R5000-. I told them I can get it for R3000- from a other company.Not going to pay R2000- more for the same thing. I requested a refund of my money and they refused.