Why must i pay for nothing

Why should i pay for nothing.On Saterday I went to buy an electricity relay at Electro Mechaninc atWoodbridge Business park.but when and on my arrival at home relay was not the size,because it was alreadyclose I had to arrangethe right size on the other places took it back to the shop and Monday I returned the relay to Electro Mechanic,The part was checked if is still in order,Joh I was so surprice when they tell me they deduct 15% from purchase price of the item.is that lawful..they told me is a handing fee..what is that?