The “in-app purchase” is a donation page where you can donate extra to the developer if you want to. It’s 100% voluntary and doesn’t unlock any extra or additional features. I’ve never donated anything beyond the $5 purchase price.

If you’re doing a search, it’s play:sub. Here is the link on iOS. https://apps.apple.com/us/app/play-sub-music-streamer/id955329386

Also, it’s a fantastic app and I use it every day.

Thanks for this. I’ll post there next time!