The lucky bags reset everyday. They are a form of progress bar, so if you dont open them on the day you reach the points required, the contents will vanish.

You might want to test your theory, but I think the comment you highlighted refered to the way you receive vouchers, which would be in a box (?). From personnal experience, I dont remember ever having more than 25 lucky bags at once. I have a tendency to overcheck my things so I dont miss anything, but everytime I forgot something like that it either showed up in my mail or vanished on reset. If anyone can confirm you can ac*ulate lucky bags, I will gladly change my stance.