This literally just happened to me. I just bought a copy of Starcraft II wings of liberty because I couldnt find my boxed copy, and when I went to the website, all I saw was heart of the swarm. I buy the key in the B/S/T thread and then when I try to attach it to my account is says I already own it.
Low and behold, I had to download heart of the swarm and launch it from there. Long story short, if anyone wants Starcraft II: Wings of Liberty for $7.50 via paypal, you let me know