The microtransactions from the mystery box are added to the store four weeks after the mystery box leaves the store. This is most likely around the fourth week of 3.13.
This thread was added on December 03, 2020, with posts from Community_Team.
Original Post
https://www.reddit.com/r/pathofexile/comments/k5lsjf/will_the_twilight_mystery_box_skins_be_available/