This is the crucial part. If the transport depends essentially on petrol, why care about the plastic? Plastic comes from petrol and is first used as a container

This is the crucial part. If the transport depends essentially on petrol, why care about the plastic? Plastic comes from petrol and is first used as a container, then to produce heat when burning it. If single-use container helps improve the logistics, then I don't think it is the main issue. I remember reading about an explanation about the sub-packaging of biscuits. Consumers would complain that a pack of biscuits contained multiple packs of biscuits itself. But it all made sense when accounting for the fact that biscuits go stale, and producing them takes more energy than storing them in smaller packages inside the main package. This all assumes a correct treatment of plastic containers. But my point is that the main issue lies with logistics, that produces lots of CO2, rather than with containers.