Keep getting charged for equipment I have never had recently.

For the last 2 months I have had to request that a $4.85 charge for a digital converter taken off? I own my modem and router. I don't have anything else in my townhome, I just plug them in the wall. When I chat with an agent they keep assuring me that I shouldn't pay it and don't worry about it. But after I paid my bill this month, there it is, it looks like I owe $4.85 still. Why can't this charge be taking off? It's very confusing. And why can't this be taken care of once and for all? On and off I have been a Comcast/Xfinity customer since the mid 1980's, for God's sake why is this occuring?