They can do cost/benefit analysis. They don't have to pay themselves 15% of the price including shipping in fees, they have good relationships with cheap overse

They can do cost/benefit analysis. They don't have to pay themselves 15% of the price including shipping in fees, they have good relationships with cheap overseas manufacturers (in China you pay literally pennies for plastic stuff in large quantities) utilizing economy of scales, they know the exact demand/sales of their competitor/"partner", probability they can convert competitor's sales into their own sales, control the visibility on their platform etc. I am sure they make larger profit than what they would get by taking a cut from a 3rd party seller. Or they are irrational, which I don't believe.