I don't think an extended debate over semantics is helpful, but virtually no one would use the word "customer" in this way. If the Wal-Mart CEO said they were going to lower prices for customers, the Coca-Cola shareholders are unlikely to be happy. Consider that Coca-Cola is selling goods to Wal-Mart for cash. That is generally not what a customer looks like.

It's common for companies to pay for retail shelf space and placement.

Acer just sent me a $20 rebate check on the monitor I bought, but they ain't my customer.

That's just not the same at all.

Well by that logic, I provide a service of funneling money into Wal-mart's bank account -- therefore Wal-mart is also my customer.

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.