great idea

Today when I was at Costco I discovered they were selling the "green" shopping bags. A 3 pack for under $4. The bags are huge and I love that I don't have to make multiple trips from the car to the house. I gave up their cardboard boxes awhile ago because they were so hard to break down for recycling. All my purchases fit in one bag and I'm thinking a weeks worth of groceries would fit in just one as well! What's proper bag etiquette? Can I bring a Costco bag into Haggen's?