Rule 356.2.b.1

Some Optional Costs specified by Passive Abilities on the card being played or another card are Non-Mandatory, and must be paid only if the player made the choice to pay them in step 2. They use the phrase "as an additional cost" and the word "may." Example: A unit has the ability "As you play me, you may discard 1 as an additional cost. If you do, reduce my cost by [2]." While playing the unit, its controller declares their intention to pay the additional cost in step 2, applies that additional cost in rule 356.2, applies the discount granted by paying that cost in rule 356.4, and discards a card to pay that additional cost in rule 357.2. See rule 363. Passive Abilities for more information.