The behavior of Albert is consistent with the law of demand.
The basic law of demand says that the higher the price of a commodity, the lower the quantity demanded; and the lower the price of a commodity, the higher the quantity demanded.
Albert went to his local store, hoping to buy a pair of Levi's for $30, however, when he got there, the price was lower at $18, he then decided to buy more than one because the price was lower. This is the law of demand taking place.