As usual it depends on the state. One state I lived in had a list of all the UPS Store/Mail Boxes Etc stores and would not let you use one of those as your physical address. They also had a physical address location element, if you did not receive mail at your physical address you had to provide a map of how to get you your "home".

Best suggestion would be to try the mail drop address first. Sometimes they want you to bring in a piece of mail delivered to that address when you apply for your DL. Again it depends on the state. YMMV.