Cost per Sale (CPS) is an online advertisement pricing system. Here, the advertiser pays the publisher on the basis of sales generated through the according advertisement.
Thus, it is a specific type of the Cost per Action (CPA) metric where advertisers only pay publishers for certain completed actions of the user. In this case the predefined action is a sale, so basically the purchase of a product or booking of a service through a user.
Since this type of mobile advertising is strictly performance-based it is very popular with advertisers. These only have to pay for a completed purchase of their product by a user. Therefore, they always receive a guaranteed value for their ad spends. Accordingly, every single dollar spent within this payment metric will earn the advertiser money through the purchase of his product. This means, there are definitely no wasted ad spends.
However, the Cost per Sale is also significantly higher than the advertiser costs within other pricing methods.
You can find a detailed overview of all pricing models available in this article.