"Snapping" carseats onto shopping carts *EDIT*
Several people have asked for alternatives to putting the cart on top, so I thought I would update with some that I've seen/used.
1) wearing the baby. I know it doesn't work for everyone, though.
2) putting the carrier in the big basket and arranging your groceries around it. You also have the top area and under the basket to use.
3) bringing along a "shopping buddy" (or older child if you have one) to push a second cart.