Surely not. A company the size of Amazon/UPS doesn't need to pay someone else to shoulder the risk of them losing $500 now and again. But it must be factored into their margins.
No it's not. If you book it under "Unplanned expenditures" then you're not insured. If you try to determine your risk in such a way that you can budget a fixed sum for coverage and reasonably expect not to exceed it (ie. not an unplanned expenditure) in the same way an insurance company would do it, they you're insured.
It's literally a line item in the budget for unplanned expenditures. Sounds ridiculous, but you are planning for the unplanned. I'm just saying calling that "insurance" dilutes the meaning of what insurance actually is, which requires a transaction between two separate companies.
Speculation: They probably force the supplier or courier to foot the bill. Maybe sack the delivery driver with a carefully vague intimation that they've been stealing?
Well, that's super-paranoid. Amazon uses major name suppliers like UPS. So it's not their employee to sack. And I doubt UPS would allow Amazon to go around randomly accusing their employees of theft.