[boxidea]

I'd bet the short obvious answer is that it doesn't cost them anything to ship small, lightweight products like this.

The plane was already flying from China to wherever, filled with AliExpress merchandise. They may have paid for the whole plane instead of just by weight. So adding this product didn't add any tangible extra weight or cost.

Then the mail carrier or whoever is already going on that route. I doubt he gets paid per package, so adding another small package is no big deal and doesn't add extra time/cost.

Unless someone is ordering 1million buttons, then the weight starts to add up. But then so does the cost of the buttons, which at that point would cover the shipping.

I'm generalizing a bit since I don't know the specifics of the shipping industry. But this makes sense to me.

[mikeash]

That works, until the plane really is full and you need a new one, or your letter carrier really is fully occupied and you need a second person.

It makes little sense to give nearly everyone free shipping, but charge every 100,000th person $50,000 because their item was the one that needed a new plane. So instead, they divide the cost across all the stuff that gets shipped so everyone pays a share. That per-item cost is typically more than what these sellers are charging, thus the questioning.

[gohrt]

There is no single optimal way to divide the cost among all the passengers. See also: airlines, and capitalist vs socialist economies.