Where are you before you go into a shop? Surely some times it's at work or home, or the car. Might help to keep a couple of fold-up bags in each of those spots. Usually habits lag these phased changes by a couple of years, but you get better at being prepared or anticipating times you'll need them.
I’ll give you an example from today. I was out for coffee and wasn’t expecting to go shopping. I walked to a book store and bought some books. They have me a plastic bag. I’d normally turn it down and just carry the books, but it was raining a little bit.
If it was important, I’d remember. I don’t think this is important so I don’t care enough to always carry a bag with me.
I'm not saying there are never situations where it's useful, just that they're uncommon and collateral damage in trying to change broader behaviour (whether by charging for bags or changing the bags on offer).
In your example where I live, they'd offer a paper bag which would cover the little bit of rain. If there was enough rain to cause trouble, people would have an umbrella.
