A trip to Costco is rarely just a trip to Costco. You walk in intending to buy paper towels and leave pushing a cart filled with Japanese barbecue sauce, fruit jerky, a 12-pack of ice cream sandwiches and, this month, an entire machine devoted to frozen drinks — you know who you are. This is the natural order of things at Costco in August — the month that combines end-of-summer clearance opportunities, back-to-school restocking, the very first Halloween arrivals, and a genuin