I've noticed! It is a good litmus test for deal pricing though, so I'm glad you're so vocal about it. To be fair though, this is the lowest this game has been by a longshot based on the research I've done. I think the next best sale on this title has been $25.
Damn, I think I'm going to have to bite with the knowledge that this is the best sale on the game so far. That 6% is what keeps me from going insane on Steam sales sometimes, so I established a guideline for myself to only buy games at 66% off or more. I've broken that guideline only once or twice in the last year and the two games I purchased were some of my most played titles (Borderlands- 250 hours PC, Borderlands 250 hours PS3) The second Borderlands 2 goes to 50% off or so, I'm gonna say "Screw the rules! I have money!"
Edit: where do you go to see the CD key for downloadable games on Amazon? I'd like to register it on Origin or whatever.