I’m speaking as someone who has the best of intentions and has bought many a reusable cotton mesh bag but can NEVER remember to bring them to the store and has a stockpile of plastic bags in my kitchen cabinet: What if the solution is a new, cool bag that’s collapsible to the size of a credit card? It not only has that style factor but would be much easier to remember–you’d leave it in your wallet all the time.
Still a bag, but IMHO solves some of the most common problems.