原文You cast the spell while Herald of Amity's second ability is resolving and still on the stack. You can't wait to cast it later in the turn. Timing restrictions based on the card's types are ignored.
原文If you cast a spell "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the spell has any mandatory additional costs, you must pay those to cast the spell.