It seems like they are "metering" the sales by making them available at a certain rate. When I first went to the play store site, it showed "sold out". Later

It seems like they are "metering" the sales by making them available at a certain rate. When I first went to the play store site, it showed "sold out". Later on, I refreshed the page randomly and it showed the price tag button which lets you order. I tried to order it, but by the time I got to the checkout page it showed my shopping cart as empty. Going back to the front page showed "sold out again". I waited a while, reloaded once in a while until I saw the price button again, and clicked through really fast to the checkout screen and I was able to purchase one. Your mileage may vary.