cart.Cart
  package.NameInPackage

So the name Cart is an interface, the function/method signatures are indicating that a Cart is being returned but being explicit about the package the interface belongs to with cart.Cart. It's made more confusing since the name cart (lower-c) is being used for both a member variable and the package name in the signature of Put, and it looks like the package name doesn't show up on an explicit line in that chapter to help clarify it.