A modern mail sack

The other day, I found this on the porch:

It turned out that it was a modern mail sack from Britain. (I remember when mail sacks used to be linen – because linen withstood abrasion so much better than any other material...)

Inside the sack was a perfectly normal package:

and inside the package was this:

I used to own this book but lost it. Now it exists in a paperback version, much cheaper than the old hardback. And the same contents!

Then I looked closer at the mailbag. It is woven of narrow strips of plastic – and it is woven in the round!
I wonder how they handle the strips to keep them flat? I can’t see a single twist anywhere – and I remember my own experience with the flat paper yarn...

And I also wonder why I got the whole sack...