Lucky Packs (sometimes called Fukubukuro) are similar to grab bags. For a set price you get a random selection of goods from the brand's shop. The total value of the goods in the lucky pack is many times significantly higher than the price lucky packs are sold for.
Lucky packs are sold for a variety of different reasons, most notably for end of the year clearance, but also on special occassions. The selection of items can range from any combination of any items in the shop, or they can be a more defined selection, such as 1 random blouse, 1 random skirt, and a number of random accessories.
You cannot pick the contents of lucky packs, but some brands will sometimes allow you to pick a size or between two different styles or colors.