A bookstore orders $1,000$ binders from its regular supplier for a total price of $$4,000,$ which represents the normal $20\%$ discount that the supplier offers to the bookstore. The supplier sends the $1,000$ binders to the bookstore, along with an invoice in the amount of $$5,000,$ stating that the state of the economy is forcing it to charge more. The bookstore sends a check for $$4,000$ marked "paid in full." The supplier deposits the check and then brings suit against the bookstore for the additional $$1,000.$

The bookstore's best argument would be:

Implied Novation.

Novation.

Promissory Estoppel.

Accord and Satisfaction.