Unexpectedly high shipping fees are the No. 1 reason consumers abandon their online purchases, according to a survey conducted for PayPal by research company comScore Inc.

According to the survey, 43 percent of consumers didn't pay for items in their shopping carts because shipping charges were too high. And when would-be buyers are hit with unexpected information at checkout, particularly if it is related to cost, two out of three fail to pay for the items in their shopping carts.

In addition, 21 percent of those who responded to the survey said they abandon their purchases because their preferred payment options weren't offered on a merchant's Web site. That's because many shoppers think it's too much trouble to search for their wallets or purses to look for another method of payment, according to the survey.

Online comparison shopping is also a common reason for checkout abandonment, according to the survey. More than one in four shoppers surveyed said that they wanted to compare items at online and offline stores before they made their purchases. More than a third of shoppers who abandon their purchases at checkout said they later returned to the merchant's Web site to complete the transaction.

The survey found that:

• 36 percent of shoppers abandoned their purchases because they felt the overall cost was higher than anticipated;
• 27 percent didn't pay for items because they wanted to comparison shop at other sites before finalizing a purchase;
• 16 percent didn't pay for items because they were unable to contact customer support to have their questions answered;
• And 14 percent didn't pay for items because they forgot their usernames and passwords for their store accounts.