As American consumers wrap up their holiday shopping sprees, ringing up an estimated $6 billion or more in online sales, some e-commerce players big and small have failed the customer service test.

Toys R Us Inc. is e-mailing some consumers, saying it will not be able to fill their online orders in time for Christmas. The company, which did not return a phone call yesterday, apologized and offered $100 gift certificates for use in its stores, but customers are fuming.

"It's just not serious business," said Lillian Argilagos, a Potomac resident who learned yesterday that gifts she had ordered from Toysrus.com for her two children on Dec. 7 would not arrive in time. "By Dec. 15, they ought to have known and should have stopped taking orders."

In recent weeks, some of the best-known Internet stores have not been able to keep up with the crush of e-mail messages and telephone calls from customers with questions about product availability, deliveries, ordering errors and returns, among other things.

Several online stores--including eToys Inc. and Toysrus.com--did not answer e-mail messages sent more than a week ago, according to an informal Washington Post survey. At Amazon.com Inc.'s busy call center yesterday, a soothing recorded voice suggested that consumers pass the time on hold by browsing the retailer's Web site for the answers to their questions.

"Just because you're a household name doesn't mean you're providing good customer service at this point," said technology analyst Herbert Tinger of FAC/Equities in Albany, N.Y.

Even before Thanksgiving, Web sites weren't performing up to customers' expectations, according to a survey by FAC/Equities. Of 89 sites, only 64 responded, and the e-mail systems of two of the sites didn't work at all.

The speediest sites belonged to smaller firms such as Ibaby.com and Omaha Steaks, as well as large stores such as Circuit City and L.L. Bean, according to FAC/Equities. Among the online merchants that did not answer e-mail were Value America, Gateway and Kmart's Bluelight.com.

If past experience holds true, online shoppers are in for more frustration after Christmas, when the time comes for returns, exchanges and refunds. Last year, the number of e-tailers failing to answer customers' messages shot up in the first quarter--when shoppers were returning gifts and asking questions about errant deliveries, according to Jupiter Communications analyst David Schatsky.

"A lot of people were having problems that they were having trouble getting the answer to," Schatsky said. "E-commerce sites just failed to come through."

Many online merchants have met the challenge, however, according to analyst Tom Casey of PricewaterhouseCoopers in Boston. Indeed, some shoppers may have discovered standouts in cyberspace.

Last week, customer representatives at Jcrew.com, CDnow Inc. and Eddiebauer.com took the time to answer questions such as "Is this sweater itchy?" and "Is this handbag suitable for a man?"

And at music retailer CDNow, a savvy customer representative named "Anthony B." advised against ordering a Christmas gift by standard shipping last week.

"I would not be able to guarantee" a timely delivery, he wrote in an e-mail message.

For most online retailers, the frenzy is over. They are closing their doors to Christmas shoppers and are no longer promising to ship in time for the holidays. Although a few Internet stores, such as Jcrew.com, will accept Christmas orders today, most e-tailers say they don't want to risk it out of fear of disappointing shoppers.

"We don't feel comfortable now," said Steve Dong, vice president of operations for CDnow, which stopped accepting Christmas orders Tuesday night. "There's a lot of stuff that can happen now with our carriers. They're overloaded right now."

At least one shipper, however, is tired of being blamed for online retailers' mistakes.

"They do it all the time," said Steve Holmes, a United Parcel Service spokesman. "What's our lesson learned from last year? That we didn't do a good job of telling people that when your package didn't arrive on time, it's most likely not because it's tied up in the UPS system."

Casey, the PricewaterhouseCoopers analyst, said the people who didn't get their online deliveries will be among the millions of consumers racing to malls across America on Friday afternoon.

"What you'll find are the typical last-minute shoppers, as well as some people who will arrive there looking more irritable because 'it wasn't in the mail,' " Casey said.

DEAR CUSTOMER, WITH APOLOGIES

Among the many overwhelmed online retailers was Toys R Us, which found itself compensating irate customers:

Dear Valued Customer:

As we approach our holiday deadline, it becomes apparent to us that even with our fulfillment center working around the clock, WE WILL NOT BE ABLE TO DELIVER YOUR ORDER BEFORE DECEMBER 25TH . . . IN THE NEXT 48 HOURS WE WILL SEND TO YOU BY OVERNIGHT EXPRESS, $100 WORTH OF FREE GEOFFREY DOLLARS that can be used to purchase additional gifts . . .

On behalf of everyone at Toysrus.com, we sincerely apologize for being unable to meet your expectations . . .

Sincere apologies,

John Barbour

CEO

Toysrus.com