The cardboard box is the least expensive crate you can purchase in Book Of Monsters. You can choose which monster to buy it for.
The cardboard box cost 500 Coins or 40 Dux and will give you one skin. It has a 94% chance of giving you a common skin, a 5% chance of giving you a rare skin, and a 1% chance of giving you a legendary skin. You can also get this on day 1 of your daily rewards.