Where milk producers have joined a duly incorporated milk producers’ association in this state and written request has been made by the individual milk producer to the milk dealer or milk company to whom they sell their milk, authorizing said milk dealer or milk company to make deductions of one cent (or specified amount) per one hundred pounds of milk delivered to the milk dealer’s or milk company’s individual creamery or milk receiving station located in this state, and when requests for such deductions have been made by milk producers to the number of seventy-five per cent or more of the total number of milk producers who deliver their milk to milk dealer’s or milk company’s individual receiving plant located in this state, in such events all milk dealers or milk companies to whom this request has been made (and authorization has been given in writing by the individual milk producer) shall make said deductions and forward all sums so deducted to a designated incorporated milk producers’ association on or before the fifteenth of the month following the month in which deductions were made, together with a statement showing number of pounds of milk delivered by each individual milk producer from which deductions were made. Milk dealers or milk companies may, however, retain five per cent of total deductions to cover cost of bookkeeping involved.