Whenever a commission merchant or dealer handling any agricultural products fails to carry out the provisions of RCW 20.01.370 as now or hereafter amended or RCW 20.01.380, whichever is applicable, it shall be prima facie evidence that the transaction involving the handling of any agricultural products between the consignor and the commission merchant or dealer was either a commission type transaction, or dealer transaction constituting an outright sale by the consignor, whichever is most favorable to the consignor. Such determination in favor of the consignor shall be based on the market price of the agricultural product in question at the time the complaint is filed against said commission merchant or dealer by the consignor: PROVIDED, That if the return to the consignor is determined most favorably on a commission basis, the total commission shall not exceed ten percent, and all other charges for handling the agricultural product in question shall be figured on the basis of the actual cost of said handling.