I thought that the Phonak charger had a place for a silica-gel dryer insert. I see pictures that show a round part with multiple small holes, which I assume is for the dryer insert.
Update: the pictures to which I referred are for the “genuine Phonak” charger. I do not see that part with the multiple small holes in the pictures of the Costco version, but it looks as though one could just drop one of those dessicant capsules in the charger anyway.