In StitcherAds, rules can be used as a stop-less mechanism by design to prevent wasted ad spend. As always, it's important to not get too aggressive with your Rules structure so that you choke out campaigns too early. Glance at this article really quickly for more information: Cost per Purchase Rule
Here's an example of a more sophisticated Rules Funnel setup where the target is a $20 CPA:
-- Rule 1 = spend > $250 + CPA > $75, pause & email
-- Rule 2 = spend > $500 + CPA > $50, pause & email
-- Rule 3 = spend > $1,000 + CPA > $30, pause & email
Live campaigns may also have ad sets that have zero purchases. In this case, the Cost per Purchase is going to remain at $0 until the first Purchase is made. Therefore, an addition rule is needed that will pause the ad set/send notifications if the number of Purchases = 0:
A Quick Note on Prospecting Campaigns...
For campaigns that exclude an audience like "Website Visitors - past 30 days", as a best practice for aligning alongside retargeting campaigns, a Rule based on Cost per Purchase might not make sense. Instead, it would be more appropriate to create rules for Cost per View Content or Cost per Add to Cart.