E-commerce could be a huge opportunity for Costco Wholesale in the years ahead.A top retail stock to own this year has undoubtedly been Costco Wholesale (COST -0.53%). Its shares are up more than 35% since January as it has been hitting new all-time highs along the way. Not only is it a stable investment given its extremely popular warehouses, but the company has also continually found ways to grow its business.You might think that with the stock now trading at more than 50 times earnings and the economy po ...