Except as otherwise provided in Section 62803, or unless the parties agree otherwise, every contract for the sale of edible nuts shall be in writing and shall state the full purchase price in a definite sum which is to be paid in accordance with the terms of the contract and, if the price is to be paid upon the basis of units or measures, the contract shall specify or describe the unit and state the full unit price. Written contracts under this section shall state the method by which the price is to be determined.