[throwbacktictac]

For example, ACME Developer could post ads that lead to a web-based signup flow (optionally collect payment info), then provide an App store link.

Until the user provides payment info (via in App or payment site), use an semi-aggressive email drip campaign to steer users to the payment website.

Some percentage of user will find (and use) the App Stores payment process. The hope is that those that due use the website make the campaign payoff in the long term.