Unfair handling fee charge

I bought a Mi box streaming stick. I installed it in my Airbnb but it did not work well. I have a WiFi extender in the flat and I’m assuming there was a conflict of some description. I packaged the contents of the box perfectly and returned it the very next day explaining why and asking for a refund. To my amazement I was told they would deduct a 15% handling fee. I have never experienced this when returning an item to other stores. I asked if they would sell the item for 15% less to the next person and was met with a blank look.I was then told to look at the back of the till slip and indeed there, in the tiniest writing it speaks of a handling fee if not returned in perfect condition. I returned it in perfect condition but was told that I’d tried it first! So AFTER you’ve paid for your item, you check the slip and find out you’ll be 15% poorer if you return it!!! Never again will I walk through the doors of Incredible Connection. Very pleased I didn’t return a more expensive item!!!!