MacGyverMacGuffin

Why in God's name are you doing this on Christmas?

The best way that I've found is with auto-renewable subscriptions. I think a lot of developers are leaving money on the table when they charge a flat fee for an app or feature.

Auto-renewable billing takes advantage of the lazy and uneducated, many of whom aren't consciously renewing their subscriptions. When you say other developers "are leaving money on the table" by not doing this, aren't you normalizing predatory billing practices and rationalizing them as business acumen?

If it's only been a year and a half of you "making six figures" off of your apps, do you really consider yourself successful? Is that gross revenue or net profit? Wouldn't you need to pass the common five-year threshold of failure of most businesses to have enough data to extrapolate long-term rates of income?

And unlike OP, I graduated.

It's nothing but self promotion.