Lucky Used Cars, using their contracts form, agreed to sell Alvin a truck for $5,000. The contract provides that it may not be modified—except by a signed writing. After entering the contract, Lucky and Alvin agreed that Lucky would include new floor mats and a bed liner for an extra $100. At delivery time, Lucky refuses to supply the new floor mats and bed liner unless Alvin pays an extra $250. 

Discuss the enforceability of the oral agreement Lucky and Alvin had.