Cost per action/sale methods require that referred visitors do more than visit the advertiser's website before the affiliate receives a commission. The advertiser must convert that visitor first. It is in the best interest of the affiliate to send the most closely targeted traffic to the advertiser as possible to increase the chance of a conversion. The risk is absorbed by the affiliate who funnels their traffic to the campaign (normally a landing page). In the case a conversion is not fired the publisher won't receive any compensation for the traffic.