The times I've experienced "prime membership" [0], Amazon generally offers "no rush shipping credits" of $1 towards "digital content" [1]. Obviously their cost basis for that is much lower than the face value. But assuming 50% discounting, 100% labor cost overhead, and using the 60 packages/hour figure in a sibling comment, only 20% of customers would have to choose the option for a worker to get a 15 minute break every 2 hours.

The popularity would also be a great indicator of how many customers resent their practices, before they simply disappear.

[0] free trials of course, because why would I pay to subject myself to a sunk cost fallacy?

[1] aka stuff that you're better off torrenting