E-retail sales from Nov. 1 to Dec. 31 increased 25% from the same period a year ago for the 50 large online retailers tracked by Chase Paymentech’s Cyber Holiday Pulse Index. The payment processor also says the number of transactions increased 37% and the average order value declined 9% during the same time frame. Chase Paymentech does not disclose which retailers it includes in the index.

Chase Paymentech attributes e-retailers’ holiday season performance primarily to consumers’ increasing shift to buying online along with heavy seasonal promotional activity. Consumers this holiday season also increased their purchases of digital media, such as music downloads and e-books, which contributed to the decline in average order values, Chase Paymentech says.

Consumers made more online purchases later in the holiday season than last year, the Cyber Holiday Pulse Index showed. Because Christmas fell on a Sunday, many e-retailers pushed shipping deadlines later than last year. Eight e-retailers ranked among the top 100 in Internet Retailer’s Top 500 Guide offered guaranteed delivery for orders placed as late as Dec. 23, according to a review by Internet Retailer. According to Dec. 23 data from the Cyber Holiday Pulse Index, the 50 retailers in the Index generated $226.1 million in sales that day, up 25.9% from $179.6 million on the comparable day in 2010.

ComScore Inc. yesterday said online shoppers in the United States spent nearly $37.2 billion during the 2011 holiday season, up 15% from the approximately $32.4 billion spent in 2010. The web measurement firm accounted for spending between Nov. 1 and Dec. 31, and tracks online purchasing behavior of 2 million consumers.

Sales at bricks-and-mortar sales grew more modestly. Kantar Retail reported today that same-store sales for 24 major chains, mainly apparel merchants, were up 3.6% in December compared with December 2010.