Running CPA campaigns in RollerAds is currently not possible.
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However, you can use the CPA Optimization feature (when choosing SmartCPC for Push or CPC for In-Page Push). CPA Optimization (Target CPA) will automatically disable underperforming areas to reach your desired CPA. In order for CPA Optimization to work, you need to set up conversion tracking using S2S postback in the “Tracking” section.