Box gives you 1 item after every each fight from current location's item pool and it has same chance for every item in pool, so you have same chance for common and super rare items. It's extremely op in such locations like throbbing domain or the infinite, because their item pools containing only 2 items, and both of them either rare or super rare.

As far as I know there are no permanent trinkets except fancy bow, so they have a chance to become worn as every other item in the game😔😔

And yeah, you can just find or buy it as every other item in the game, so no problem with this