Comment by smackfu

Usually it is that the sellers are actually different, like one is Amazon itself, and the other is a third-party.

So you have a product that is priced like: Amazon.com: $14.99 with $5 shipping (free for Prime) 3rd Party: $12.99 with $3 shipping.

Amazon will show Prime members the first one, and non-Prime members the 2nd one.