I can’t picture a ‘jug of jug milk’. Isn’t it hard to carry without spilling, or is this not really a jug?

A picture is worth a thousand words, dxb.

Here's a Milk Jug recycled as a bird feeder.

http://users.aristotle.net/~shicks/birds/Feeder.html

Some larger jugs don't have a handle inset into the jug which takes away from capacity. Instead they have a plastic handle hanging from the neck of the Jug.

"Milk Stores" are really just convenience stores that got started selling milk at discount prices, probably as a loss leader.

I have heard that there is a company in Michigan that recycles milk jugs into all-weather patio furniture. It won't wear, chip or fade, and you can leave it outside all year long.