The history of orthodontics has been intimately linked with the history of dentistry for more than 2000 years. Dentistry had its origins as a part of medicine.
Addressing an envelope to a post office box address is just like addressing envelopes to any other type of address. Of course, before you can address the envelope, you need the name of the recipient, the post office box number and the city, state and ZIP code.
FedEx has a similar last mile by UPSPS service as well. It is important to note that USPS Priority Mail to PO Boxes can be VERY VERY expensive once the packages gets over a few pounds...UPS Sure Post is much cheaper and allows you to delivery to a PO Box (though UPS otherwise can't accept delivery to a PO Box).