For inspecting and certifying the grade, quality, or condition of grapes or plums the Secretary shall cause to be collected a reasonable fee which shall, as nearly as may be, cover the cost of the service rendered: Provided, That when cooperative arrangements satisfactory to the Secretary, or his designated representative, for carrying out the purposes of this chapter cannot be made the fees collected hereunder in such cases shall be available until expended to defray the cost of the service rendered, and in such cases the limitations on the amounts expended for the purchase and maintenance of motor-propelled passenger-carrying vehicles shall not be applicable: Provided further, That certificates issued by the authorized agents of the United States Department of Agriculture shall be received in all courts of the United States as prima facie evidence of the truth of the statements therein contained.