What's the cheapest thing you can buy one million of?

Really? Maybe I’m missing something, but can you order 1 million [something] and have them send that to you, as opposed to, say 900,000 of that same [something]?

I’m not sure who you are responding to, but for resistors, you can absolutely order exactly a million of them. One of the highest stocked resistors at Digikey can only be ordered in multiples of 100,000, in fact:

You can get the same resistor in single units, though.

And remarkably, Digikey carries over a million different types of resistor. You could order one of each and have a million distinct manufactured objects.

Right.
But, think of how many Akro bins you would need to store them!

But then, how is that cheaper than staples? Or is it the fact you can only buy staples and multiple staples, rather than individually?

Yes.
You can’t go to a store and ask for “a staple."

All right. A different reading of the OP than mine, but understandable one.

Depends who’s ordering. I assume it’s not unusual for a computer maker to order, say, a million memory chips for the factory. And… you can buy one of them by itself.

During the run-up in silver prices, much of the Canadian silver coinage (dimes and quarters) dsiappeared from circulation. I have a small collection of silver coins that I hoarded every time I ran across one in the last few decades. Apparently during the high, people were melting silver down - which was illegal. So there was the loophole that it was not illegal to melt Canadian coins in the USA, but border guard would turn back anyone tryng to enter the USA with a large collection of Canadian coins. One article described a border guard commenting on the fellow who hid a large bag of coins in the engine compartment, but it broke on the way - leaving a silver trail of coins across the bridge in Niagara Falls… and back to the Canadian side.

The company Paperworks used to sell a box of 100 paper clips for $0.01. Not sure if they still do (it was a loss-leader). So $10,000 for a million boxes or $100 for a million paper clips.