That's illegal. It's fine to add incentives to make your products more lucrative for the business, but it's illegal to pay a company to sell your product over another -- either implicitly or explicitly.
That is not illegal. It happens in contracts everyday. Several examples for you. One is the iPhone itself. ATT paid Apple for exclusivity in the US. Yes, VZW initially turned it down, but after it was released, VZW would have offered the iPhone years ago. The reason they couldn't? The ATT exclusivity contract. In this case, ATT paid to explicitly deny others the right to sell the phone.
Second example is your local grocery store. Think that display on the endcap of XYZ product is there by accident or because the store decides to push more of it? It is there because the manufacturer or distributor paid to put it there. Ever get frustrated that your favorite Product A is at the bottom of the shelf, forcing you to bend over uncomfortably to get it while its competitor, Product B is at eye level and easy to reach? The maker of Product B paid the store to put the product at eye level, and the store is implicitly promoting one product over another. This is called product placement, and is alive and well in every retail chain.
Check out your local professional stadium. Chances are that the stadium only sells Coke or Pepsi products, but not both. Why? Coke or Pepsi pays the stadium operator to offer their beverage products only. Ever wonder why you can't buy a John Deere product at Home Depot but you can at Lowe's?
The reasons for all of these are that one organization is paying another to promote one product or service over another. It happens every day, and is not illegal.