At the moment the magic lamp switches between free shipping and $5 off (the switch may be every week, every few days or some random schedule). I believe you can only use it once per cycle (the expiration date being the cycle). I used the free shipping, so now the magic lamp has disapeeared for me. It should reappear 5/31 when the current magic lamp expires, or something like that.
Once you sign in, you see the magic lamp link to click on, tucked in around the financing and geek squad ads. If you are not signed in, I don't believe the magic lamp offer is available.