The Director may establish a classification of goods and services, for convenience of Patent and Trademark Office administration, but not to limit or extend the applicant’s or registrant’s rights. The applicant may apply to register a mark for any or all of the goods or services on or in connection with which he or she is using or has a bona fide intention to use the mark in commerce: Provided, That if the Director by regulation permits the filing of an application for the registration of a mark for goods or services which fall within a plurality of classes, a fee equaling the sum of the fees for filing an application in each class shall be paid, and the Director may issue a single certificate of registration for such mark.