This offer popped up for me RIGHT after I finished the second Hans battle. So I'm guessing they're making it available for people who have just enough time to defeat Hans but who won't have enough time left to obtain ANY of his epic items. Because, yeesh, at $20, if you are going to have enough time to obtain even just some of his items, it might be better to wait until the last minute and compare the cost of buying just the remaining items with gems. The combo pack offer seems to last right up until the event ends, so your options will be open the whole time.