Use 'afterthought' to indicate a casual, sometimes unimportant, addition to a conversation or text. He added her name to the invitation list as an afterthought.
Pro Tip 2/3
Sudden Realization
'Afterthought' can express a sudden realization that comes after the main event or decision. Oh, she might need a ride too, I guess that was an afterthought.
Pro Tip 3/3
Non-priority
'Afterthought' often suggests that something is not a priority and was considered only later. The decorations were an afterthought; the party was all about the music.