I was granted full custody of my 8 year old son in 2016. The mother did not complete assignments that was deemed for us to do. The case was closed.

  Mr. Adams:      So if you were granted "full custody"  in 2016, then parernity must have already been established.  I am not sure what a "paternity case letter" is and how you received it.  If it is a modification action, you have to be served by a sheriff or process server.  If it is a Motion on something existing it can be sent to you by mail.  But you would have to be more specific as to what the document says.      Perhaps she is trying to modify the prior court order so she has time with your son?  If so, she has a big burden of showing that there has been some unanticipated change in circumstances since the court's prior ruling,   and that the change will, ultimately, be in your son's best interest.  It's a big burden on her.   You might benefit  from a consultation with an attorney to explain why you are to go to court, and then you can take it from there.  Just because she filed something does not mean she is going to get the relief she requests.  Your son's best interest is paramount, and I am sure you have been a good father to him.   Best of luck, Cindy S. Vova Law Offices of Cindy S. Vova, P.A. Broward/Boca Raton 954-316-3496/561-962-2785 info@vovalaw.com