Offers to ship furnaces to Zippy

Explain your answers in as much detail as possible. For each answer, state the law (including all of the relevant elements of the law), state the relevant facts, apply the law to the facts, and give your opinion as to what the outcome should be.
Yellow Heating Inc. offers to ship furnaces to Zippy Inc. for $4,500 cash. Zippy's CEO e-mails back, "Great, I accept, as long as we can make that payable in
three weeks, not tomorrow." What are the elements of a valid acceptance? Was this a valid acceptance and why?