Temporary Item Box is variant of Item Box where the items contained has time limits. These items will disappear once they reached the time limit. Because of this, it is cheap.
Example :
Temporary Item Box
5000 sm
18% Common (2 weeks time limit)
25% Rare (1 week 4 days time limit)
33% Epic (6 days time limit)
20% Legendary (4 days time limit)
4% Mythical (1 day time limit)

I would reduce the mythicals time to 12 hours, more would be better for the players, but 12 hours makes it more attractive for the developers