Even if it seems to be secure, shopping with the infant seat on top of the cart is never safe. It makes the cart top-heavy. Babies in seats can and have toppled off the top of the cart or had the whole cart tip over on them, leading to skull fractures and other severe injuries. Although it might appear that the seat “clicks” onto the cart securely, it doesn’t stop it from being top-heavy, and attaching the seat to something it’s not designed to attach to can damage the locking mechanism and prevent it from holding the seat into the base in a crash. It seems like everybody does this and therefore it must be safe, but it isn’t. Don’t do it.
On the following pages, I'll explain what you should do instead!