You pay $X and if you get sales from those ads you pay $X + $Y because we proved the ads made you money.

That's the problem. In your original scenario the advertising platform isn't proving ads resulted in sales. They're showing the ads after the sale occurs.

You said the company would be self-reporting their sales to the advertiser so that they'd have to pay the advertiser more money. But companies often run ad campaigns on multiple platforms and even multiple campaigns on one platform. They're not going to pay extra multiple times to multiple companies because they made a single sale.