I spend more for mac apps every year than the year before. And I do out it outside of the app store (mostly). Obviously HN crowd isn't representative of the general populace but this is my limited anecdotal experience.
how?
But if I was in your position, my response would be to do the best job I can to educate my users about this and hold their hand through the process as much as possible. If your competitors face the same challenges, doing this well could be a real competitive advantage.
A prompt at every instance is just fear mongering.