Billing charge

My account is being charged an extra $100 for an installation fee or a technician service.

When I picked up my router and tv boxes, the service lady’s notified me it would be a free installation if i called to get help setting up the internet router. 
I called the xfinity customer service line and I made sure to ask and confirm there would be no charge added, as I needed assistance connecting the internet.
I am not paying the $100 I was never informed about.

when the technician left he left no receipt stating any charges nor was there any mention of being charged a fee, the whole time he was at my residence. 
I do not agree I should pay this $100!!!!