It would seem crazy for an ad agency to implement that through an IAP where Apple takes $3k off the top (I assume that 30% rate still applies here?) when you're dealing with high-paying clients that could be told "go to our website to purchase"
Eh, not really. The benefit of in-app purchases is you can make a button that users can press that's probably already hooked up to their credit card so they can make purchases without the ask of getting them to type in their credit card number, which is a more conscious action that gives them plenty of time to think about whether or not they /really/ want to make that purchase. Amazon's one-click checkout, ShopPay, etc are not just for user convenience!
Comparatively, a high-profile client spending tens of thousands of dollars in ads isn't doing so on impulse, they're doing it cause it's their job. They'll open up a browser and punch in some banking details if it's the only way to get those ads out.
