Unfair handling fee charges

I am highly frustrated, annoyed disappointed by my experience with hifi corp. I bought a product online which does not work. I am now being charged handling fee of 15%. How do i know that it works or not without opening it. Bear in mind online shopping is for your convenience but i had to go back to the shop so many times to eventually get a refund of course less the handling fee. On top of that i was called rude for venting out my frustrations