The annual license fee to conduct the business of an employment agent, to receive applications for employment, to hire or contract with persons for employment shall be $20, except that the annual license fee for an agency or registry for the employment of registered professional nurses, practical nurses or undergraduate nurses shall be $20; provided, however, that the provisions of this section shall not be applicable to any such agency or registry operated by a registered professional nurses association or any district subdivision thereof for the exclusive benefit of its registrants and not for profit.