* Mobile shopping on rise, Apple's iPad leads the way
* Many people spend less and buy fewer items online
* Shipping costs could eat into retailers' profits
CHICAGO/SAN FRANCISCO, Nov 24 (Reuters) - Online shopping has grown sharply so far this holiday shopping season after U.S. retailers pushed out a slew of mobile and Internet deals that lured consumers before Cyber Monday, traditionally the biggest day for e-commerce in America.
While it remains to be seen if the gains on Thanksgiving Day and Black Friday can be sustained, the latest estimates show shoppers scooping up early special offers via their smart phones and tablets, the way they used to hunt for bargains in brick-and-mortar stores.
Online sales increased 17.4 percent on Thanksgiving and 20.7 percent on Black Friday, compared with 2011, according to IBM Smarter Commerce, a unit of International Business Machines Corp that analyzes transactions from 500 U.S. retailers.
Black Friday, the day after Thanksgiving, had traditionally been the kickoff to the holiday season for stores. This year, retailers such as Walmart and Target made their biggest push ever with special offers during the holiday itself.
"The Thanksgiving creep revitalized the thrill for people," said Wendy Liebmann, chief executive of WSL Strategic Retail. "It got people excited to go out. But it pulled a lot of sales forward."
In-store sales on Thanksgiving jumped 71 percent and more than tripled between 6 p.m. and midnight from a year earlier, according to Chase Paymentech, a leading credit card processor.
On Black Friday, in-store sales dropped 7 percent, Chase Paymentech noted.
Cyber Monday, which follows the long holiday weekend, has been the biggest day for online shopping in recent years, as workers return to their office computers.
Now, armed with mobile devices, particularly Apple Inc's iPad and iPhone, shoppers are no longer waiting.
Shoppers are also using the devices to track down the lowest prices. The average order value on Black Friday declined by 4.7 percent to $181.22, and the average number of items per order dropped 12 percent to 5.6, according to IBM.
"Our Black Friday numbers were a little softer than Thanksgiving," said Eric Best, chief executive of Mercent, which helps merchants sell more on websites including Amazon.com, ebay.com and Google Inc's online shopping program. "That tells me there are timing shifts happening this holiday season."
Client sales on Black Friday rose 23 percent from last year, while Thanksgiving sales rose 32 percent year over year, Mercent estimated.
"What we don't know is whether this is a zero-sum game or whether there is some benefit to retailers by broadening the holiday shopping window," Best said. "There is risk that real growth in retail for the entire holiday may be overestimated based on these early numbers."
Scot Wingo, chief executive of e-commerce company ChannelAdvisor, calls the early online shopping season "Cyber 5," referring to Thanksgiving, Black Friday, Saturday, Sunday and Cyber Monday.
"The online players are definitely participating more on Thanksgiving and Black Friday, so Cyber Monday is being shared with other holidays now," Wingo said.
ChannelAdvisor clients saw same-store sales on Amazon.com jump 35 percent on Thanksgiving and 38.3 percent on Black Friday, compared with a year earlier.
Client same-store sales on eBay's online marketplace rose 27 percent on Thanksgiving and 31 percent on Black Friday, compared with a year earlier. Last year's year-over-year growth was about 15 percent for eBay, according to ChannelAdvisor.
EBay usually grows more slowly early in the holiday season and then ramps up later as retailers run out of hot products and shoppers turn to eBay sellers for those items.
EBay's strong start this year is driven partly by the company's success in attracting mobile shoppers through shopping apps on smart phones and tablets, Wingo said.
EBay said on Saturday that the volume of mobile transactions on its online marketplace jumped 153 percent on Black Friday, compared with a year earlier. PayPal, the payments division of eBay, saw Black Friday volume almost triple, year over year.
Purchases from mobile devices accounted for 16.3 percent of online sales on Black Friday, up from 9.8 percent in 2011 and 3.2 percent in 2010, according to IBM data. Apple Inc's iPads and iPhones led the charge, with owners using the devices to browse and buy on retailer websites.
SHOPPERS 'WANT TO SPEND'
Overall, the percentage of sales from online shopping is expected to rise. But while retailers bring in more revenue from online shopping, they may also face some higher costs in terms of shipping a larger number of packages to more shoppers.
Among tablets, iPads were the clear leader, generating 88.3 percent of traffic to retailers' sites on Friday, followed by Barnes & Noble Inc's Nook with 3.1 percent, Amazon.com Inc's Kindle with 2.4 percent and the Samsung Galaxy with 1.8 percent, IBM said.
"It's either going to be a much bigger holiday or people are shopping earlier in the season," said Wingo. "We won't know until later in the season."
Stores continued to use discounts to lure shoppers on Saturday, with Aeropostale Inc discounting items as much as 70 percent after a storewide 60 percent discount on Friday.
Rival American Eagle Outfitters Inc continued its two-day sale at 40 percent off, and Gap Inc's namesake chain was offering 60 percent discounts for the entire weekend.
"The discounts were reasonable but didn't take your breath away," said Liebmann. "Retailers are being cautious."
The real tests for retailers will be their levels of discounting over the entire season as well as the amount of online sales this weekend.
"American shoppers want to spend. Just give them a reason to come out," said Walter Stackow, portfolio manager with Manning & Napier. "They're trained to hold out for deals as Christmas gets closer."
The Garden State Plaza Mall in Paramus, New Jersey, appeared to be crowded on Saturday, although much quieter than on Black Friday, clerks at several stores said.
Some people were just starting their holiday shopping.
John Dunlap of East Orange, New Jersey, bought bedding at Macy's and said he skipped Black Friday as it is "too crazy" and not worth it unless someone is shopping for electronics. He said he would shop throughout the season only if he found good deals.
"They have to give good discounts because of the economy," Dunlap said.
Vanessa Crenshaw, a 45-year-old accountant shopping at JC Penney, said that if stores pulled back on discounts, she would go elsewhere.
"You can always find a deal, someone will have a deal," she said.