It's evident that you don't know what you're talking about. That is not a service in any meaningful way. A service is any kind of action performed for compensation. That action cannot be the actual act of compensating. Does Walmart give you money for giving them money? No.

This translates to Amazon. They are selling the rights to distribute one's books on their platform, and in return take 30%. Amazon provides a service, the bookseller pays them money. (In this case, that money is zero, but that's beside the point. In general, the seller is Amazon's customer.)

I was replying to the comment that claimed Coca-Cola was Wal-mart's customer instead of their supplier -- I wasn't taking a stance on Amazon.