In my searching, I found this bag that seems like it has potential. The Pack-N-Tote Grocery Cart helper bag is designed with both the function of holding groceries, as well as how to transport them. Two bags can fit nicely into a normal shopping cart. There are velcro tabs to secure it in place. The bags have an accordian design to fold down, so you can leave them in your car. When you get to the grocery store, grab a cart, open up and secure the bags, do your shopping, and then easily transport them back to the car and return the empty cart. There is a hard bottom which keeps the bags from tipping over while driving. The bag is also adjustable to the size of the load (each bag holds up to 40 pounds which is quite a lot!) $8.95. Available here
What type of reusable bags do you guys use? Share in the comments!