The secretary shall by regulation establish a classification of goods and services for convenience of administration of the Trademark Act but not to limit or extend the applicant’s or registrant’s rights. A single application for registration of a mark may include any or all goods upon which, or services with which, the mark is actually being used indicating the appropriate class or classes of goods or services. When a single application includes goods or services that fall within multiple classes, the secretary shall require payment of twenty-five dollars ($25.00) for each class. As far as practical the classification of goods and services should conform to the classification adopted by the United States patent and trademark office.