Charged after cancellation, told they couldn't un-charge me

I canceled my Xfinity service on March 11 (my current apartment has wifi already and I am poor) and received a bill on March 13 which included $100 for a technician visit that I was previously told would be free (or I would not have agreed to it) and an additional charge of 34.52 (probably to return the Gateway? Although I was told on the phone when I cancelled service, first to return it and then "don't bother returning it".)

I went on March 26 to return the Gateway. They accepted it and gave me a number to call for Billing. There I explained that I was being charged for a service I was told would be free, and that I had returned the equipment and so should not be charged for it (never mind that I was told I didn't have to return it). was told that I was in the right place to resolve it, that they would get the charges taken off, but oh wait, they couldn't, because I cancelled my account and they couldn't get in to remove the charges. So apparently, even if I should not be charged for these things (and I shouldn't) they somehow just can't stop billing me. It's just out of Xfinity's hands.

Xfinity, can you not bill me $95 over what is, most charitably, a very serious unintentional problem with how you set up billing and accounts? I need that money for diapers for my baby, medical expenses for my own disability. For wheelchair transportation to the store which I don't even know if I had to go to to return the Gateway. 

Incidentally, all the customer service people, all the employees that I spoke to after I cancelled were great. None of them were responsible for this problem.