If you're sure you want one - yeah order and pay now. I normally don't sell things I don't actually have, but in this case I want to make sure to get enough shirts without getting too many. The profit margin is very slim on these, so if there even a handful left over I'll end up losing money. I'm okay losing a little because this is a site promotion, but I can't afford four shirts for each of you to ensure having the right size.
Bottom line - I'd appreciate it if you ordered now, but it's okay not to and you'll still be able to get a shirt. The pre-orders will ship two days before everyone else - there's a little incentive.