The US Post Office has a calculator on their website that figures out how much you can reduce your carbon footprint by shipping online. It is pretty cool. I figured out that if I drove two miles to the Post Office five times a month that I would save the planet from 230.7 pounds of CO2 and save $41.35 in fuel.
In addition, shippers that ship online save an average of 3.5% on Priority Mail and get free Delivery Confirmation! So, if I shipped just one Priority Mail envelope for each of those trips, I would save 15% on postage($.65 on Delivery Confirmation and $.05 on a one-pound envelope)
So, save yourself some money and help the planet by shipping online.