can you apply your knowledge of contract law principles in analyzing and interpreting this issue relating to contract law?

Situation

Sara needs her car repaired before she leaves for a business trip on December 15. She takes it to Speedy Repair, where she agrees to pay $500, and Speedy Repair agrees to have her car ready for December 14. On December 12, Speedy Repair telephones Sara to tell her that the repairs are behind schedule and it does not think it can have the car ready by December 14 as promised. Sara is frantic and offers them an additional $500 to have the car ready for December 14. 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 14, as well as being shiny and clean. Sara then refuses to pay the extra $500. Is Speedy Repair entitled to the additional $500? Explain.