CPA or CPE – Cost per action or cost per event. In this case, an advertiser pays only if a predetermined valid action occurred. This is a relatively safe and low-risk way to buy media because the advertiser pays only if a specific event has been completed. Many media companies are not likely to sell media this way because of the risks associated with it. If no one buys, they make no money – rules are simple. In the CPA model, the publisher takes the maximum risk as income is dependent on good conversion rates.