Should I get a mocha or a tea?

So this is kinda weird. I've got a job interview tomorrow morning ... at a Starbucks. I realize that this guy conducting the interview is probably keen to have them somewhere he is comfortable, and where he can stay caffeinated, but this just comes off as ... kinda weird. And totally unprofessional. As it is, I was actually considering showing up in serious casual, including flip flops. But no, my cooler head prevailed, and instead, I'll be wearing my Toms with my jeans, and a button up (maybe). Because even though it's at a Starbucks, it's still a job interview (or is it?).
The job is the first one I mentioned a couple weeks ago, where the company was looking for someone with "social media awesomeness," to post for companies to their own Twitter, Facebook, etc., and other social media sites. As when I applied for this job, I still consider it to be a perfect fit for me, and as my husband told me, "I can give them an excellent character reference that you are on social media all the time."
The position will be at their main office for a couple weeks, but then can be an at-home job. This again, works out incredibly well, as I'm looking forward to spending my mornings at Starbucks on their wi-fi, and my afternoons at the library next door to Sydney's school. Or maybe the other way around. Or maybe, if they have the room and the people are cool, I'll stay at the office.
I'm trying to not get too ahead of myself here (though the previous paragraph disputes that), and this is my first interview in a while, but I tend to interview well. And there's nowhere that I'm more comfortable than in a Starbucks.