Amazon has offered free two-day shipping for Prime members since 2005. As Amazon rolls out a one-day shipping guarantee for its 100 million Prime members, Amazon Flex drivers help solve the company's last mile problem. CNBC spoke to these on-demand contract workers all over the country to find out what it's really like to deliver for Amazon. Watch the video to hear about the pay, pace of work, and how Amazon Flex works.

Is Amazon Prime worth it? For many members, free shipping is a key Amazon Prime benefit that factors into their decision to pay $119 per year for the service. As Amazon starts offering Prime members the added benefit of one-day shipping they're working with on-demand contract drivers to help with this especially labor-intensive and expensive hand-delivery.

We spent a day with Omar Montes, a Flex driver in the San Francisco Bay Area. Omar helped a friend deliver around 46 packages in 3½ hours, for $105. “These are like the good days that make you want to continue doing Amazon. But, you know, there’s obviously bad days too,” Montes said.

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What It's Like To Be An Amazon Flex Delivery Driver