One counterintuitive savings strategy that can work for frequent online shoppers: Pay to get free shipping.

Barnes & Noble, announced to consumers in early May that the $25 annual fee for its membership club now includes free one- to three-day shipping on all orders, in addition to its previously offered exclusive coupons and discounts. The bookseller is just the latest in a long line of online retailers to offer a buy-in shipping club; others include Amazon.com, Sears, Overstock.com and Wal-Mart .

Retailers hope an offer of unlimited free shipping will entice frequent online shoppers to consolidate their business, and maybe cut their in-store shopping with competitors, too, says Scott Testa, an assistant professor of business administration at Cabrini College in Radnor, Pa. It s a way to stay competitive, he says. The extra revenue more than offsets any loss in shipping fees.

Whether free-shipping clubs are a good value depends on your buying habits. Before you whip out a credit card to pay for an annual membership, consider these six factors:

Cost

Shipping clubs range from $10 to $80 a year. Use online order history from the retailer in question, as well as its competitors, to calculate potential savings:

* Amazon.com: $79 per year. Amazon Prime offers free two-day shipping and $3.99 one-day shipping. Free 30-day trial membership. (Regularly, rates for standard shipping run $1.99 to $7.99, plus a per-item charge of 99 cents. Heavier items carry an extra charge of 59 cents to 85 cents per pound.)

* Barnes & Noble: $25 per year. Barnes & Noble Membership offers free one- to three-day shipping. Members also get exclusive coupons and discounts, including 20% off all adult hard-cover books and 10% off most other items. (Regularly, rates for standard shipping run $1.99 to $3, depending on the item, plus a per-item charge of 99 cents.)

* Overstock.com: $20 per year. Members of Club O receive free shipping on every order (regularly $2.95). They also get a 5% discount on most items, and exclusive coupons and discounts.

* Sears: $79 per year. ShipVantage offers free standard shipping, $0.99 for two-day shipping and $3.99 one-day shipping at Sears and Kmart. (Regularly, rates start at $6.95, with a $10 surcharge for two-day shipping and a $15 surcharge for one-day shipping.)

* SmartBargains.com: $10 per year. Members of the Smart Shoppers Club get $2.95 standard shipping (regularly $9.95) and $4.95 three- to six-day shipping (regularly $11.95). They also have 48-hour advance access to new arrivals and markdowns, as well as exclusive sales.

* Wal-Mart: $29 per year. Site-to-Store Express readies items for in-store pickup in three to five days, instead of the site s free Site-to-Store option of seven to 10 days.

Store availability

With a store nearby, free or low-cost expedited shipping loses some of its value, says Luke Knowles, the founder of FreeShipping.org, a deal-tracking site. If you really need it that fast, you could just go to your local store and get it, he says. (Keep in mind that shipping offers don t factor in processing time, which can add an extra day or two.)

Other benefits

Clubs can be more valuable if they offer more than just free shipping, Testa says. Barnes & Noble members can save at least 10% on most items, and often receive exclusive coupons for biggest discounts, while Overstock s Club O members get a 5% discount on most items.

Exclusions

Shipping clubs don t cover everything on the site. Sears, for example, doesn t include items that are delivered rather than mailed, such as furniture and appliances. Barnes & Noble and Amazon won t cover items shipped by individual sellers or other third parties, while Overstock s 5% Club O discount doesn t extend to books, DVDs, CDs and other media.

Renewal

Memberships usually auto-renew, says Linda Sherry, a spokeswoman for Consumer Action, an advocacy group. Check the policy so that you know whether you need to closely monitor your account, or if the company will warn you before renewal time approaches.

Other free-shipping options

A lot of these stores will give you free shipping anyway if you hit a certain threshold, Knowles says. Before you pay, see if there s another way to save without paying. Sears and Wal-Mart offer free site-to-store shipping. Amazon routinely offers free shipping on orders of $25 or more. Barnes & Noble does too, and also lets consumers reserve in-store stock online for pickup.