If you want an environmentally responsible way to shop online, take a look at
EVO.com. This site lists over 100,000 products and services from more than 1,200 vendors that all claim to be "green." There's everything listed here from hybrid autos to bedding, batteries to organic flowers. Products and retailers are rated by environmental impact, energy consumption, use of sustainable materials and more for a "greenness" rating (represented by a number of leaves) to make sure that you really are getting a green product. The site does not actually sell products, but offers photos prices and descriptions that are linked to the retailer's site.
Sign up for an account and EVO will plant a tree on your behalf.