While it cannot be used as an option for "your physical residence.... in legal documents" -- voter registration or the DMV -- the USPS does offer "Street Addressing Service." It provides a street address you can use with UPS, FedEx, and others. P.O. Box 59 would be 500 Main Street #59 (for a Post Office located at 500 Main Street). The service is free. (I just filled out the form two days ago.)