Guys, it doesn't matter what carrier you say you want. If anything you'd want the AT&T model just because it has Band 30, but it doesn't matter. Someone at the Genius Bar even showed me because I was curious, they're in the system as '6S Band 30 Model' and '6S Non-Band 30 Model'. If you bought it full price, it's an unlocked model, and carrier name doesn't even show up in the device profile, it's just 'Unlocked'.
So literally, all you have to do is walk into an Apple Store, find someone to help you:
'Hi, I would like to buy an iPhone at full price.'
'Sure, what carrier?'
Then say AT&T if you want the Band 30 model, or literally anyone else if you don't care or they don't have it. It truly doesn't matter.
You only have to worry about locked phones when you're buying from AT&T, Sprint, and I think T-Mobile still if you buy from them rather than Apple. I stopped buying iPhones from carriers a year ago because I didn't want to deal with their bull**** anymore.