356.4.f. Discounts can reduce additional costs, including to 0. 356.4.f.1. An optional additional cost was "paid" if the player made the decision to pay it. It doesn't matter how much the player actually paid. Example: Clockwork Keeper is a unit that costs 2 Energy and 0 Power and says "As you play me, you may pay [C] as an additional cost. If you do, draw