What are some Facebook Ads API partners that offer CPA with very low or no minimum ad spends?

The first two bid types available were CPC and CPM so to buy one a CPA the PMD was generally offering the risk, However Facebook has now enabled CPA bidding (Cost Per Action (CPA) Ads). This obviously reduces that risk and anyone with a self-service platform can grant you access to it if they have implemented these bid types. Of course, the specifics of them vary by PMD. CPA bidding is not yet available for native Facebook (and still carries some limitations) so you'll need a PMD to get in on CPA bidding as of the time of this writing.