Still on my parent's plan as well (gotta keep dat grandfathered unlimited data) and am an authorized user, so I'm concerned about this as well since I live about 4 hours drive from my parents. Also complicating this is the fact that my shipping/billing address is different from my license as I just moved last month.. but I have a little card that the DMV sent me with my new info on it to keep with my ID. I plan on going in there with: my ID, DMV card with new address on it, dad's social, print-out of my order information, and a print-out of the cell phone bill. They better give me my phone.
Also found it odd that in 2012, I was able to get my iPhone 5 shipped to my address, and when we got my wife a 5S last year, it had to be shipped to my parent's. She was just joining our plan, so I chalked it up to being a new line. When I ordered this morning, the 6 for my upgrade couldn't be shipped to me. Glad I was planning in-store pick-up anyway.
what you can do is make a account password. that way u dont need the social. when you go in u just need the password and ID.