charged for returned equipment

This is the same problem that many other users are experiencing.  We cancelled our Xfinity service, we returned our equipment to a store. The store could not give me a receipt because they had already wiped the equipment from the account but I took a couple of screen shots and there is some kind of processing number in the web address. They couldn't remove the charge because the system had to update first. Or that's what they said anyway. 

They said to go home and call Xfinity after an hour so Xfinity could credit us back the $320 they were charging us for unreturned equipment.  I called Xfinity today and they said there was nothing they could do about it.  They advised me to go to the store and get them to take the charges off of our account...  The store is an hour away, one way. The only phone number that I can find is the main 800# customer service number for Xfinity so I can't call them.  I'm afraid that if I go to the store they will send me down another rabbit hole.  I'm beginning to feel quite like a ping pong ball here...

Surely Xfinity customer service can see that there is no equipment listed on our account and credit us back the $302?  Could you please help me with this issue?  This is a lot of money and it scares me to think I might have to pay it.