Offer often comes with strings attached.
Online shoppers are attracted to three things: low prices, no sales tax and free shipping.
This Christmas season more than 90 percent of online merchants are offering some sort of no cost shipping, that's up from 85 percent last year according to the National Retail Federation.
Online shopper Tara Northcutt says she only buys from online retailers who don't charge for shipping.
"If they are offering free shipping they are advertising that right away," says Northcutt.
The last few days this mother of 3 boys has shopped sites from Toys R Us to Kohls and never paid an extra dime for shipping. But at times she did buy something extra to get free shipping.
"My purchase would be $30 to $40 and so there would be that extra $10 I'd have to spend," said Northcutt.
Many sites have a minimum requirement to qualify for free shipping from $25 to $100.
"I think consumers will often buy an extra item to move over that and try to save money," said San Diego State marketing professor George Belch. "I think free shipping can induce consumers to spend more than they originally thought they might do."
According to the Internet marketing research firm ComScore people getting free shipping spend more money per transaction.
In the 2010 holiday season, consumers getting free shipping spent about 15 percent more than those who did not.
Did Tara Northcutt buy anything she wouldn't normally buy to get the free shipping?
"No, I was trying to be very, very careful," said Northcutt, "that's something I can see happening to myself."