The One Trip Grocery Bag Holder is a great idea that should have come about twenty years ago. 
Basically, it is just a simple plastic handle with a clip mechanism that lets you add plastic bags to it – thereby allowing you to carry up to 50lbs in one hand (as long as your muscles are up to the challenge) without cutting off the circulation to your fingers… ingenious.
Unfortunately, in my neck of the woods most people no longer use plastic bags thanks to punitive measurements put into place by local governments.