OnAir decoder cancelation fee

I received a call from OnAir TV sales person on Saturday and she pitched for a new decoder that she claimed it comes with Showmax and Netflix and I will only pay R250. 00 monthly. When I try to connect the decoder it wasn't connecting. I called multichoice and they explained to me that I still need to pay my multichoice account and OnAir TV account but it wasn't explained to me in that manner by the On Air TV consultant. I canceled the deal today as I received the decoder yesterday. Know I've been charged a R350. 00 cancelation fee because they claim that the box is opened. Ofcourse I had to open the box for me to gain excess of the items.... Know I want to know why am I being charged because the fault is not with me as a consumer but the fault is with their sales person... I request for my R350. 00 to be reversed back to my account and next time they need to train their consultants to pitch their sales with honesty and stop ****ming people....these people are ****mers I want my money back