It’s in the bag
Canvas bags can be picked up cheap, but you can’t avoid them — people are always handing out canvas bags full of goodies (and marketing materials) at all sorts of events and giveaways.
There’s no excuse to be using plastic bags anymore, and certainly no excuse to be buying a new one every time you head out for groceries. Keep a canvas bag or two in your handbag, backpack, or satchel, so it’s handy for whenever you might need it, and get into the habit of putting it back as soon as you empty it.