Refund request due to incorrect part supp****

I purchased pistons from Alert Boksburg, I good faith I took the parts to the engineers and the pistons where pressed only for me to find out that it was the wrong pistons! I went back today 28 June and they refused to replace the pistons as they were now pressed, however They supp**** me with the wrong part! I now had to pay another R2000 odd. I WANT A REFUND! AS this was due to their negligence!