Liesa, Shroud of Dusk

Liesa's first ability isn't optional. You can't choose to pay mana to cover the "commander tax," even if you have that mana available.

Liesa's first ability applies only to the "commander tax." If there are any other additional costs to cast Liesa, those costs must be paid as normal.

Liesa's last ability will resolve before the spell that caused it to trigger.