I've done a fair bit of work on the highways, and in the forestry. I can't say I've seen woodland badly littered, but then I am in a thinly populated rural area. The highways, on the other hand, are still littered with all sorts of fast food packaging, cans, crisp packets, sweet packets, and so on. So, OK, there are no supermarket bags any more, but quite honestly, it hardly makes any difference. At least a discerning litter bug may have taken the time to put all the litter in the bag and chuck it out the window, making it easier to pick up, but now it all comes out separately.
Compared with the rest of the plastic that goes to landfill, or ends up in the sea, reducing the number of plastic bags, right at the most inconvenient point in their lifecycle, will achieve sweet nothing.
If it was the supermarket or the food producer taking the financial and inconvenience hit for it, I'd say nothing, but charging the customer for it just smacks of money grabbing.
And why oh why are they not allowed to supply cardboard boxes to carry goods out? As said above somewhere, they are commercial waste, what an unfortunate coincidence......