The clock is ticking away for online shoppers ordering holiday gifts.

Standard shipping deadlines for arrival by Christmas are swiftly approaching: You have just hours left at some retailers and a day or so at others. (KSS) shoppers have until 11 p.m. EST. today while Drugstore.com gives until Dec. 20 at 3 p.m. EST. (Unless you pay for expedited shipping, you ve missed the boat at a handful of e-tailers, including Buy.com, LEGO Shop and (HD).)

Those deadlines are on retailers minds in this lackluster holiday season. Anxious for one last grab at your online business, many stores are offering a final round of no-charge standard shipping deals during today s Free Shipping Day. Among the offers: free two- to three-day shipping on all orders at (AAPL), free shipping on all orders at (SKS) and (Overstock.com) and free shipping on everything but media at (OSTK). More than half are offering free shipping on all orders, says Luke Knowles, the founder of Free Shipping Day and the year-round deal site FreeShipping.org.

While a free shipping offer can be tempting, keep in mind that retailers are well aware that it can make the difference between buying or not, says Catherine Williams, the vice president of financial literacy for Money Management International, which oversees nonprofit credit-counseling agencies. They are looking for every opportunity to bring you into their virtual stores and fill your cart, she says.

Before you buy, use these tips to make sure you re getting the best deal:

Weigh cart filler

Minimum order requirements for free shipping entice you to spend more than you otherwise would, Williams says. Somehow you ll be induced to keep adding to your cart, she says. It doesn t make sense to buy an extra $25 in products you wouldn t otherwise buy just to save $10.

Compare coupon codes

The first free shipping promotion you come across may not be the best deal. Consider the full range of offers available via user-driven coupon sites including RetailMeNot.com, FreeShipping.org and CouponCabin.com. Stores often simultaneously offer promotions at different price levels. For example, teen clothing retailer Delia s currently has free shipping on orders of $75 or more (code DWAW75 ) or $25 or more (code DXRW25 ).

In fact, free shipping may not be the best deal at all if a retailer allows you to enter just one code at checkout, Knowles says. Broader dollar- or percentage-off codes may be more valuable. New York & Company code 3857 gets you free standard shipping (regularly $12.95) on an order of $50 or more through Dec. 31. But code 4254, which offers 20% off orders through Dec. 24, provides the bigger discount if you re spending more than $65.

Ship purchases to the store

Are you prepared to brave Christmas Eve crowds to save on shipping? Some stores, including (WMT), (RSH) and Payless let you buy an item online and have it shipped free to the nearest store instead of paying to ship it to your home. (For more details, click here You can use Web-only coupons and in many cases still get your order in time for the holidays. (Wal-Mart s ship-to-store deadline is Dec. 17.)

Check shipping deadlines

An offer of free shipping does not necessarily go hand in hand with a guarantee of holiday delivery, says Manish Rathi, a co-founder of consumer gadget review site Retrevo.comYou can also check the DealNews.com roundup of deadlines here and Retrevo s listings here