Follow the first post here:
http://store.apple.com/us/question/...uying-phones-from-a-carrier/Q2C44C2PXCCYT9DKK
I bought my iPhone 6+ via Verizon's website because I was tired of in store reps pushing me to buy overpriced crap that I could get on Amazon for 1/2 the price.
So, first, I got my Serial Number and my IMEI number - wrote them down on a post it. Called 1-800-275-2273 - the automated assistant confirmed my device and knew my name right away. When a human came online, I just said I wanted AppleCare+ - gave a credit card # and was charged $99 for AppleCare+. They give you a confirmation number that they want you to write down and read back to them. Hours later, I got an email with my papers.
I've had AppleCare+ before. When I shattered my iPhone 5 screen, I got a new one for $50 (I think). It's worth it.