Xfinity ripping customers exiting their service

I cancelled xfinity last year, and i didnt get a chance to return device immediately as i was busy moving. By the time i got a chance to return both router and tv box in the store, they already charged me 270 dollars. I asked the store lady on how to get the refund and she asked me to call xfinity customer service. After explaining the situation, customer service rep asked me to speak with my credit card company as it was still in pending state. I spoke with my chase customer service representative and shared the receipt that xfinity store lady gave to me and chase helped me to dispute the charge. Recently i got a mail from debt collection agency to pay it or they will take action. Why do i need to pay i already returned the devices. Horrible experience, never ever going to use xfinity in my life, currently on verizon and its so much better despite a little higher charge.