I'm pretty sure it was just the website that can't add a line, in store they can right?
As for canceling I think i have an upper hand considering two iphone charges are pending on my account when i ordered one... i'm a bit pissed off right now.
I just called ATT and they said the double charge was a problem with my bank that i have to talk to them about.. But they also said that when my phone comes in I can just tell them I want a refund and they'll put the money back in my account