Billed after cancelling Xfinity

Hi, I cancelled my Xfinity services on 25 June.  I received a confirmation email that day.  I paid a bill on 5 July, which I expected to be my last.  I received an email on 14 July billing me for what looked like another month of service, due 5 August, even though I had cancelled my service during the previous billing period.  The Xfinity assistant -- both on chat and on phone -- are stubbornly unhelpful (no offense), so I would appreciate any guidance here.  The email I received also said I would be sent instructions on returning my Xfinity Flex, but I have received no email.  I shouldn't be charged for services Xfinity hasn't cancelled after my request.  Thank you in advance.