I built a wireless highlights & annotations export service for Kobo e-readers earlier this year [1]. Had a free tier limited to 20 exports but wasn't sure how to price it beyond that, so I just left it. Recently a user reached out asking if I'd settled on pricing and how they could pay for unlimited exports! That jolted me into coming up with a price, and now I'm finally getting Stripe integrated :)
[1] - https://highlights.email