U.S. online sales this holiday shopping season are up 15 percent compared to last year, after what may have been the busiest week of the season, said research firm comScore on Sunday. Shoppers have spent $30.9 billion online from Nov. 1 through Dec. 16, up from $26.9 billion at the same point last year, said the Reston, Va., company, which tracks Web use. Online sales surpassed $1 billion on four days last week. Total sales for the week climbed 15 percent to $6.31 billion compared to last year.