In my case, I returned three relatively expensive items in a year and I got a warning email from them. Googled the text of the mail and it turns out it's the first step to getting your Prime account banned, and based on the contract they probably wouldn't have to pro-rate the subscription fees.
They know all your purchases, home location, work location (most likely as people deliver to work all the time), and all your returns, as starters. How do they actually make use of it to upsell, etc.? Their recommendation system.