Online retailers are cautiously upbeat after unexpectedly strong holiday sales earlier this week, though some say the gains won't prevent flat growth rates for the year, according to comScore.

Total online sales were up 15 percent to $846 million on Cyber Monday, the first working day after the Thanksgiving weekend, the Reston, Va.-based Internet research firm reported. For the weekend, sales grew 13 percent over last year, the report said.

"We saw a higher-than-average order value and a 20 percent year-over-year increase in sales," said David Duong, CEO of Shoe Metro, a San Diego, Calif.-based online shoe store.

Like most retailers, Duong said he started the week with low expectations given the slowing economy and dismal retail projections.

James Bendle of Baby Earth, an online children's clothing store based in Round Rock, Texas, attributed the gains to a value-based marketing blitz that encouraged consumers to start spending again.

"It appears, at least in our market, consumers may have clamped down more severely in November in order to be able to give their children the best holiday they can in December," Bendle said.

The spike follows a two percent decline in holiday shopping sales leading up to Thanksgiving, compared to steady growth of up to 20 percent over the same period in recent years, said Andrew Lipsman, senior manager of industry analysis at comScore.

Despite the sudden upturn, comScore's forecast for online sales throughout the holiday season remains flat, Lipsman said.

Still, for retailers like Bendle, early gains have renewed hope for better year-end sales.

"A respectable start to the holiday shopping season for us means we may see performance exceeding the targets that were set in recent weeks," he said.