Poor customer service.

My sister bought earphones at Mr price on Saturday and she immediately saw that they don't work for her phone. She tried to return them and she was told that she can't. The return policy on the back of the slip does not mention that earphones cannot be replaced. It lists a few items but earphones are not there or cellphone accessories. So when we buy cellular products are we supposed to keep our fingers crossed that they work for us because we won't be able to return them if they don't work? What is she supposed to do with a product that doesn't work for her?