Like any people trying to be more environmentally conscious, I like to carry reusable bags in my purses. One of my favorite bags for groceries is the string bag. Not only is it stylish (oh so french chic), but it is also washable, expandable, and nondescript. Totally affordable as well.
You can buy four bags as a set, with options for long handles/totes. Go here. REDUCE REUSE RECYCLE.