Wrong part fitted - No refund

I received a quote for an extension for my trailer with a tailgate R4940 and my friends who does not have a tailgate R4610. Only to be charged R5240, R300 more a few days later when I had the extension fitted . So the first issue is that Venter does not honor their quotes. A few days later I start packing for my Dec holiday only to discover that I cannot use my tailgate on the extension because they fitted the gas rider to the tailgate!!! I needed the tailgate so I disconnected the gas rider, which in turn made the top lid extremely heavy. Once back from holiday I return my trailer to Venter Princess Crossing, spoke to Rory who first helped me, who passed me onto Daniel who said that he told me that the tailgate would not work with the conversion (not extension) that I had fitted extra. Then why would I have bought the extension with a tailgate then? At an extra cost of R330 for something I cannot even use. Venter should have advised me to rather go for the Elite extension instead, or am I wrong? If Daniel had told me as he said above, then I would have decided otherwise, he has no proof thereof. And why is this not indicated in the 2 quotes I received? I said in that case I want the correct extension fitted and a refund. Daniel spoke to his boss, Mr Bell who refuses to honor their mistake. So I have paid an extra R630 for an extension with a tailgate I cannot use and I was overcharged. I feel this is daylight robbery, especially from a brand such as Venter who take you for a ride knowing that you have nowhere else to go when purchasing their products.