The clock is ticking: Read on for holiday shipping deadlines and free offers

The biggest obstacle keeping holiday shoppers from buying more online? Shipping charges, according to a recent National Retail Federation survey. It makes sense: Pesky freight fees can become a serious drag on your budget, especially if you don't order early, and they're also a major negative reminder of the difference between shopping online and in person.

Understandably, then, the same survey found that free shipping is the most popular online promotion - and retailers are trying to respond. Fifty-seven percent of online retailers plan to offer free shipping without conditions this season, and even more - nearly four out of five - will offer free shipping with some strings attached. With retailers that ordinarily charge for shipping, the promotion can lead to real savings; some others that offer free shipping all the time seem to build it into the price of their items. Shoe and fashion megastore Zappos.com, for instance, offers free shipping and returns, but often prices its merchandise higher than the same items at other stores.

Because prices increase as Dec. 25th draws nearer, it's more than just helpful to keep track of shipping deadlines and deals - it can save you money and aggravation. Consumer Reports' Tod "Tightwad" Marks has put together a guide to shipping deadlines for various retailers and delivery methods here, including information about the U.S. Postal Service, UPS and FedEx. He notes that most free shipping offers apply to standard ground shipping, which will get an item to its destination in anywhere from three to 12 days, plus the day or two it takes to process a product and get it out of the warehouse.

Dec. 17 is the last day to qualify for Amazon.com's free Super Saver Shipping, while the deadline for the U.S. Postal Service Priority Mail is Dec. 21.

Another resource is FreeShipping.org, which compiles and lists free and reduced shipping offers from an array of retailers. The site also will be holding Free Shipping Day on the 17th, a week from today, with the goal of giving shoppers one last chance to order gifts that can be delivered by Christmas Eve. The site hopes to have 500 merchants participating, including plenty of big names. But let's hope they clean up some of the logos currently displayed on a promotional Web site: Defunct electronics retailer Circuit City is among the brands listed.