When you pressed the button, you know what you were buying and how much it cost, because you're the one who set it up.
As for how to determine whether the purchase went through or not, an LED would light/flash on the button to indicate success/failure, and you'd receive an email with more detailed information such as the item being purchased, its cost, and the address where it will be shipped. If the button press was a mistake, you could simply cancel the order.
When they discontinued the service, I put all the buttons in a cabinet drawer hoping that they could be "jailbroken" to act as an arbitrary IoT button in the future (like the official Amazon IoT button[1]), but as far as I can tell this never came to fruition (barring a few exceptions) and they're basically doomed to e-waste.
And not a bad idea, just one that didn't play out.
I certainly wouldn't call it 'so wrong'.