In conjunction with the installation and implementation of all electronic tolling under N.H. Rev. Stat. § 237:51, the department shall make available an optional system of anonymous transponders, allowing customers to anonymously fund and use a transponder, provided the transponder is properly funded and mounted prior to use. The transponders shall be available at public locations that allow the purchaser to remain anonymous. The department shall retain the least amount of information necessary for auditing purposes on anonymous transponder transactions.