The agency may charge reasonable fees for guarantee and insurance to students obtaining or who have obtained loans under this chapter. The agency may charge up to eighteen percent of accumulated principal and interest if the loans are in default under section 15-62.1-06. The fees collected under this section must be available to defray costs of administering the guarantee loan program. Fees in excess of the amount required to pay the cost of administering the program must be deposited in the reserve fund. The authority of the agency to charge     reasonable fees under this section applies to all borrowers and cosigners for loans obtained under this chapter whenever the underlying note was executed and subject to the note providing for payments of fees and collections costs.