Bought a gift online? Good luck returning it

$ave Me: Holiday gift returns
$ave Me: Holiday gift returns

Free shipping has given online retailers a huge boost this holiday season. But shoppers looking to return an item might find that doing so costs both racks up financial as well as time costs.

Online sales have been booming this year, with spending from Thanksgiving through Dec. 15 up 21 percent year over year, to $19.22 billion, according to comScore. But online returns could take a bite out of that tally. United Parcel Service has said it expects a 15 percent increase in holiday-purchase returns, while FedEx projects a 28 percent bump relative to its annual volume.

Many shoppers view easy returns as a key benefit to buying online.

"What we're seeing is the convenience of the return policy is becoming important in holiday shopping," said a FedEx spokesman. According to a forthcoming study from the shipper, 41 percent of consumers said they are more likely to buy an item online if the e-tailer offers free returns.

(Read more: Holiday gifts that may end up on the return pile)

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Some Web retailers have been making an effort to streamline the process, said Kevon Hills, director of research for StellaService, which rates online retailers' customer service. Free returns have become more common, and retailers may even include adhesive return labels with the original purchase.

"There are 15 that we consider to have perfect returns," including Nordstrom and Saks, he said.

Time from return to refund has also shortened, with better retailers offering a turnaround of under seven days. Amazon, Walmart Stores and Wayfair are among the companies offering "instant returns" on some purchases, Hills said, with money credited even before the item has reached the warehouse.

But online returns can still be a struggle in other cases. A number of retailers take days to approve a return before sending a shipping label, and they add costly restocking and return shipping fees.

(Read more: How to navigate tougher return policies)

Shoppers may also find that retailers consider online orders and store purchases two entirely different kinds of transactions.

"Don't assume that the online policy is the same as the brick-and-mortar policy," said Edgar Dworsky, founder of "It might be tougher; it might be more lenient."

Some retailers, including Sports Authority, don't allow items ordered online to be returned to a store, he added.

Here are four of the retailers with the most generous online policies:

Costco: The warehouse club offers free return shipping and will even refund shipping charges paid on the initial order. There's no return deadline on most items, with the exception of electronics, which must be returned within 90 days.

L.L. Bean: There's no return deadline at L.L. Bean, whose open-ended policy states that shoppers can "return anything purchased from us at any time." Items can be returned at any store, but only store-brand cardholders get free shipping for mailed returns. Everyone else eats $6.50 for the return.

Nordstrom: The company not only offers free return shipping but provides a printable online form and label. That way, customers can mail back gifts and other items not purchased online if doing so is more convenient than getting to the store. Its open-ended policy allows for returns on a case-by-case basis, with a special hotline for help with oversize or perishable goods and other unusual items. The only catch: Nordstrom Rack purchases must be returned within 30 days. Shoppers have a full 365 days from the date of purchase to return items, with free return shipping. No wardrobing, though: Items must be unworn and in their original packaging.

(Read more: Receive an awful gift? Return policies are now tighter)

Four of the retailers with tight policies:

GameStop: Shoppers have a 30-day window from the date on the packing slip to return most items, which must be unopened. (Opened, defective items can be exchanged for another of the same item within 30 days.) The policy on returning Web orders in store is vague, saying "certain items" may be returned, but also that "store return policies may vary and are subject to manager approval." but an extended holiday return policy is in place, allowing customers to return unopened merchandise with a receipt from Oct. 1 and Dec. 24 as late as Jan. 15, 2014.

GameStop said a spokesperson was not available for comment. It has more than three dozen specialized policies that vary by category and sale type, some more lenient than others. The site's standard return policy allows returns for a refund or replacement within 30 days, and tacks on a 15 percent restocking fee when a refund is requested. Customers also pay for return shipping, even on defective items. did not respond to requests for comment.

Sports Authority: On the plus side, an extended holiday return policy will let customers send back items ordered between Nov. 1 and Dec. 31 as late as Feb. 20, 2014. But online purchases cannot be returned to stores, and mailing items back costs $6.50 on each order.

Sports Authority declined to comment. Gilt's policy varies by the type and cost of the item. Most "nonsized" items, including handbags and toys, are not returnable at all. Clothing, footwear and other returnable items priced at $199.99 or less can be returned only for site credit; pricier items can be returned for credit, or a refund less $7.95 shipping. In either case, items must be returned within 21 days of the ship date.

ilt spokeswoman said the site's policy strives to be as flexible as possible, with return availability noted on each item's page and again at checkout.

By CNBC's Kelli B. Grant. Follow her on Twitter @kelligrant.