If it's Prime, it's 'free'. They will often point out that it's cheaper from other sources but often the shipping costs make up the difference. Also the third party sources don't have the world by the balls so the delivery time is a week to ten days rather than two days.
I've gotten some obvious Chinese knock-offs from Amazon sellers; one email and the problem is resolved. usually they tell me to keep the crap, refund the price, and kick the seller off their marketplace.
Hey, I'm an American and I want instant two day gratification, not waiting for a slow boat to China so I'll forget why I wanted it by the time it arrives.
I have gotten some stuff from eBay, generally strange stuff like laptop keyboards that normally don't show up for sale.