Employees, including retired employees, of a county, other than a city and county, employing more than 20,000 persons in addition to any other purposes authorized in this article may also authorize deductions to be made from their salaries, wages, or retirement allowances for the payment of dues in, or for any other service provided by, any bona fide organization whose membership is comprised, in whole or in part, of employees of such county, other than a city and county, and employees of such organization and which has as one of its objectives improvements in the terms or conditions of employment for the advancement of the welfare of such employees.