An article on GreenBiz.com offers suggestions for reducing CO2 emissions.

Shipping documents electronically can save money and CO2 emissions.  For example, airlines began creating digital records of reservations rather than paper tickets a decade ago. As a result, they have reduced operational costs from eliminating paper, envelopes, and shipping; improved customer service and satisfaction; and significantly reduced their carbon emissions. Global electronic sales in the airline industry now account for more than $400 billion a year.