Even before more than 2,000 retailers got together to create Free Shipping Day 2011 on Friday, Dec. 16, there have been so many free shipping offers this holiday shopping season than even Santa’s elves couldn’t keep up with them.

With FedEx, UPS and the United States Postal Service (USPS) expecting to deliver millions of packages before the last driver heads home for a holiday dinner, how can retailers afford to offer this early present to their customers?

One reason may be from the cuts in fuel costs and other changes to promote energy efficiency that some of the shippers have made, even as FedEx and UPS have added fuel surcharges. (The USPS has a price cap on some