The credit card I used was enough if am not mistaken for the Apple guy to find my ready to pick up order - then he asked for a photo ID (though I have the confirmation email in my iP4, just in case).
The credit card should be enough. I think a lot of people are just showing up to the stores without having their orders go through and never receiving a confirmation. A paper email is better, that way you can wave it around and slam it down on the table when your throwing your fit.