Bolide di Lusso

If an exiled card has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

If you cast a card "without paying its mana cost," you can't pay any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, those must be paid to cast the card.

If you cast the instant or sorcery card, you do so as part of the resolution of the triggered ability. You can't wait to cast them later in the turn. Timing restrictions based on a card's type are ignored.