Mystery Bag, Mystery Sack, Lucky Bag
noun
The luck sack is a mystery bag. These are bags that are sold during the New Year's holiday. They contain various goods that are sold for a certain price. You don't know what's in them, but you know the minimum value of it at least. These are known as mystery sacks as well.
ふくぶくろ
The reading is 福 and 袋, put together. Also, watch out for the rendaku.
元旦に、福袋を買う列に並ぶため、妹と私は朝の四時に起きました。
On New Year’s Day my sister and I woke up 4 a.m. to line up to buy Lucky Bags.