Charged 15% for returning a wrong item

I am so upset right now. I will never put my foot in Hi Corp West rand Lifestyle again. Yesterday I bought a cartridge for my printer. The assistant who helped me advise me that it will benefit me to buy the XL size, because it will last longer. I asked him twice if the cartridge will fit into my printer, he said yes. When I got home to fit the cartridge it didn't fit. When I returned it, I was told I am being charged 15% for opening the package. Really????? It took them another 30min to refund me my money. I am not impressed at all.