I've added in-app-purchase to MindfulGoals in October. I've granted all existing users free Pro accounts for being early adopters so no paying customers were acquired.
In December, I added a few features: dark mode, reward tracking, and deadline tracking, and launched on ProductHunt.
As part of the launch, I posted the page to my personal social network profiles: LinkedIn, Facebook, Twitter, email and SMS, asking my friends to help upvote. One of my friends became the first paying user. About a week after the ProductHunt launch, I found that a few users have subscribed to the monthly plan.
So this validates that the product is helpful to them in some way, and keeps me motivated to continue to improve it for my customers.