Do not read on if you haven't finished it
I got a bit antsy having to write things like 'that moment' or 'this makes sense now' without actually getting into specifics. So what did you think of the ending and what do you think the post credit part was all about?
I personally see it this way:
The first time they revisit the baptism Booker refuses the Baptism
Therefore the 2nd Baptism they visit is the one where he would've of otherwise committed to and where he'd become Comstock however instead of following the route they kill Booker at that point.
Therefore all future timelines have a Booker that dies at the Baptism or a Booker that walks away from the Baptism meaning Comstock can no longer possibly exist. Therefore the moment after the credits would be the Booker who is still down on his luck with Anna however he'd never encounter Comstock and is able to live his life with his child.
ALSO I'm going to go out and say it, the best scenes in the game are
The Beach Boys Quartet
The song in the basement of the bar with the acoustic guitar and small child
NEW YORKS BURNING
The pinky scene
And I love the explanation they gave for the modern music in the game, I'm glad they addressed that :D