Rule 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 1." A player controls a card that says "Units you play cost [A] less." That player plays Clockwork Keeper and chooses to pay the optional additional cost of [C]. They will draw a card, even though the optional additional cost was reduced to 0.