My company (among other things) routes print-on-demand orders to various print companies. Some of their APIs have mechanisms to ensure idempotency, some don't. The last time I pressed the issue, I was asked - and I quote - "Can't you just send the order only once?"

The thing is, having a print company that gets the printing part right is more important than having one that gets the API right. I use them anyway, and accept the risk that there will very occasionally be duplicate orders. At least in my business, it's just tshirts.

A few months ago I bought a fairly expensive cordless vacuum from hoover.com. I was charged once, but two of them arrived. I suspect I know why.