cr:5.4.4b

An alternative-cost ability is a play-static ability that replaces one or more asset-costs and/or effect-costs to play the source card. Alternative-cost abilities are typically written in the format “You may [COST] rather than pay this’s [BASE],” where COST is the alternative cost to be paid instead of the cards BASE asset-costs and/or effect-costs. Alternative-cost abilities are optional and must be declared when playing a card.[5.1.3] An alternative cost has no effect on any additional costs that must be paid or modifications to the asset-cost.