Warehouse workers and union representatives said Prime Day meant extra hours in sometimes sweltering temperatures and a heightened risk of injuries.

If you’re a Prime member, you can save up to $100 on the waterproof e-readers.

Amazon.com Inc took a big gamble this year by expanding its annual Prime Day summer sale to four days from two, betting the extension would give shoppers more time to navigate the millions of deals on its sprawling web store.