I think bags would have to cost $10-$20 before the most people started remembering to bring one to the shop. People simply don't think far enough ahead to plan

I think bags would have to cost $10-$20 before the most people started remembering to bring one to the shop. People simply don't think far enough ahead to plan to take a bag to the shop, and prefer to 'pay for convenience'. Plenty of people pay $30 to avoid having to turn their oven on to cook something.