Hello Zanzibaran,
To go standard post more than half the weight of the items need to be bound printed matter, or the package needs to weigh less than 1 pound. The pawn set makes up more than half the weight of the upcoming order so it needs to be shipped through priority mail. Sending the other books through standard post is often cheaper than sending them with the pawns through priority mail. Keep in mind we don't put the final form factors in for products until we have the products in the warehouse so this can change when we are ready to generate the order.