Not OP, but it's pretty obvious. At $1-$2 (US) per cubic foot, 4-8 packages per cu. ft. is 20¢ apiece. A shipping container is about 2400 cu. ft., more for hi-cubes.
Pretty common to ship 20mt of metal in containers-we process $8B in raw commodities a year like this. Commodity traders don’t pay these prices to ship though, these are high value manufacturing (cpg, tech) prices. Commodity firms still ship enough volume to pay sub $2k a can.
containers vary a lot in their construction and materials, right? open-top railway car containers are not the same as common trans-continental containers, and refrigerated units are different again. All of those and more are in daily use in vast numbers, worldwide.
I believe that the trucking portion of handling the container has weight limits; the machinery that handles the loading and unloading have their weight limits; the ship and the way it is loaded from bow to stern has weight limits, and a container itself has weight limits. Somewhat analogous to freeway overpass heights, the tendency over time is that the various limits harmonize with each other and you get typical shipping practices.
A shipping container full of raw cotton is indeed quite different than one full of "iron ore" but actually, consumer electronics and various car parts, their load limits, are more commonly restricted in the way they are loaded and handled.
