text: If you cast a card “without paying its mana cost,” the value of any X in the card’s mana cost must be 0. You can’t pay any alternative costs for that card. On the other hand, if the card has optional additional costs (such as kicker or replicate), you may pay those when you cast the card. If the card has mandatory additional costs (such as Fling does), you must pay those when you cast the card.