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.

In 2006, the United States Postal Service alone emitted nearly 5 million tons of CO2.  Businesses can reduce the impact of mailing by examining document processing for employees, customers, and vendors. Start by asking if sending a physical document is needed.   Recipients often prefer electronic shipping because it is timelier and receiving requires less effort.  

E-mail or fax can be alternatives to mailing for almost any business correspondence — from shareholder reports to signed legal documents.  If confirmation is required, select the "return receipt" option in an email client. If printing and shipping physical correspondence is necessary, print double-sided on recycled paper and ship by boat, rail, or ground. Only ship overnight or two-day for emergencies. Shipping by air emits 20 times more CO2 than ground, 50 times more than rail, and 150 times more than water. Be sure to use a shipping company that practices disciplined load balancing and efficient route planning.  

Finally, scrutinize cost effectiveness of printing and sending unsolicited material, especially catalogs enclosed in orders placed electronically. Check for duplicate names or address variations, cease sending when orders don't materialize, or mimic good internet practices with an easy 'unsubscribe' capability.