Problem
1) Buyer and Seller enter into a contract in which Seller will ship 1,000 widgets, priced at $1 per widget) to Buyer every month for two years.
2) The contract includes the following CONDITION: If Buyer's payment for a shipment is not received by Seller within 48 hours of Buyer having received any shipment, Seller has the right to cancel the entire contract.
3) In the third month of the first year, Seller received Buyer's payment for that month's shipment 49 hours after Buyer received the shipment.
4) Seller canceled the entire contract.
5) Buyer sued Seller for breach of contract.
What concept of Contract Law BEST supports Buyer's argument that Seller should not be allowed to cancel the entire two-year contract because of such a small-scale failure?