Packages and groceries. I've never seen one, because it's more common in more densely populated areas. Basically, they're cutting costs by avoiding UPS and FedEx when they can, using a quasi-Uber model
The earnings details are in the FAQ: https://flex.amazon.com/faq
It does seem fairer than DoorDash and Uber, since there's a guaranteed minimum hourly rate, though you're still on the hook for self-employmenmt tax and vehicular depreciation.