Costco’s once-slumping e-commerce channel is now racing ahead. The big-box retailer says e-commerce comparable sales rose about 18% in the five weeks ending Dec. 31, accelerating from a 9.9% increase in the four weeks ended Nov. 26 and a 3.7% rise in the month ended Oct. 23. Costco bucked the trend of declining online sales in its latest quarterly report, helped by shoppers ramping up their purchases of discretionary items from Costco’s website.