Hunter Farms contracted with Petrolia Grain & Feed Company, a Canadian company, to purchase a large supply of the farm herbicide Sencor from Petrolia for resale. Petrolia learned from the U.S. Customs Service that the import duty for the Sencor would be 5 percent but that the final rate could be determined only upon an inspection of the Sencor at the time of importation. Petrolia forwarded this information to Hunter. Meanwhile, Hunter employed F. W. Myers & Company, an import broker, to assist in moving the herbicide through customs by drafting the necessary papers. When customs later determined that certain chemicals in the herbicide, not listed on its label, would increase the customs duty from $30,000 to $128,000, Myers paid the additional amount under protest and turned to Hunter for indemnification. Explain what Myers would have to prove to recover from Hunter.