Most plastic waste is from weekly groceries. No one in their right mind would make a special trip to take the junk back, simply take a black sack with the previous week's garbage when next going shopping. It would be no additional cost and minimal inconvenience.

It's how recycling used to work. There were big skips for glass, paper, textiles etc in most supermarket car parks. They were well utilised by shoppers before entering the store. Before that we used to return the empties for reuse within the store.

If a campaign of returns caused supermarkets to restrict suppliers, and create less in the first place, I would do so diligently.