Sara needs her car repaired before she leaves for a business trip on December 15th. Sara primarily uses the car for personal use.

She takes it to Speedy Repair, where she agrees to pay $500 and they agree to have her car ready for December 14th. On December 12th, Speedy Repair telephones Sara to tell her that the repairs are behind schedule and they do not think they can have the car ready by December 14thas promised. Sara is frantic and offers them an additional $500 to have the car ready for December 14th. Speedy Repair agrees and offers to throw in a car wash and wax "for good business relations". Speedy Repair's mechanics work overtime and the car is ready by December 14th, as well as being shiny and clean. Sara refuses to pay the extra $500.

Is Speedy Repair entitled to the additional $500? Explain.