As far as I'm aware,As a season ticket holder you get the home friendly games thrown in.This has always been the case in the past I have always just shown my ticket and have never been asked to pay. The only exception used to be the E&B games where we were considered to be the the away side.
I think for the Charlton game you get in free but need to get a ticket in advance if you want to sit. Seasons tickets will hopefully be coming out soon with some sort of explanation. Alternatively we could email the club I suppose.