Are they psychic?
It's pretty impressive how online merchants can pinpoint the sort of things you're interested in. Take, for example, these two emails I got from Amazon.com.
Subject: Your Amazon.com order of "Canon EF 50mm f/1.8 II Camera Lens" has shipped!March 11:
Subject: Amazon.com recommends "Canon EF 50mm f/1.8 II Camera Lens" and more