bill from collection agency on closed account

I received a letter from the Credit Protection Association, LP stating I had an overdue account from Comcast Communications in the amount of $99.99. Several things are inaccurate:

~the amount...I closed my original account (I moved) with Comcast for $141.86

~the address...I changed my address with Comcast in February 2008.

~their letter states this is the 2nd communication they have sent (to the wrong address!). All my other mail to my previous address has been forwarded...how come this is the first communication I have received?

~I spoke with Comcast this morning and was informed that I had a "credit" of

$4.33.

I have written the Credit Protection agency, from their website address, and expecting a reply.