Products they act as a seller for, and products they act as a producer for.
When I'm buying item X and it's not up to par, Amazon's response is invariably "just send it on back and we'll refund you." At one point I complained I'm not taking a half-day off work to get this $5 item in the mail and they said never mind, just keep it, and we'll refund you anyway.
Awesome.
OTOH, they have a $10/mo (or so) a la carte book service, labelled "Prime Books," with a "free" subtitle (or it was - I haven't checked back recently.) Clicking on it was auto-enrolling. I didn't find out for a few months that they'd apparently one-click-enrolled me in a recurring monthly bill. It took me 3 hours on the phone to get them to partially refund it, and it ended with me closing my account then and there. Despite having been a member since they more or less "opened their doors," and a good chunk of my household spending going through their portal, refunding money from their own service was a line in the sand for them.