Emma, who owns a flower shop in Mukilteo, Washington, contracts with Luke to supply flowers for an upcoming wedding for $2,000. A week before the wedding, Luke informs Emma that he will not be able to supply the flowers unless she agrees to pay an additional $500 due to a sudden increase in flower prices. Under pressure, Emma agrees to the additional payment. After the wedding, Emma refuses to pay the extra $500, arguing that there was already an existing contract. Can Luke enforce the agreement for the additional $500?