Mengniu brand ordinary pure milk has two kinds of packaging, one is plastic bag and the other is paper bag. The milkman said the paper bag would be thicker. Is that so?

These are two different packaging forms. Paper packaging is called tetra pak. The difference between these two packaging forms will have different requirements for processing technology. What you said is thicker, but the milk packaged by Tetra Pak is fresher. Processing and packaging time is short. The price difference is due to the different packaging costs. Tetra Pak has applied for a patent for its packaging. In China, only they can provide this kind of packaging, so the price is ridiculously high, and the difference is passed on to consumers.