Full version
Buying and restoring
One purchase, €2.99, valid on Mac and iPhone. No subscription.
Buying

From the purchase window, or under Settings, Data, section Purchase. The App Store handles it; NoteBay never sees payment details.
On both devices
It is a universal purchase: bought on the Mac means unlocked on the iPhone, and the other way round. Family Sharing is on, so the purchase covers the people in your family group as well.
Restoring
After a reinstall or on a new device: Restore purchase in the purchase window, or the same place in the settings. It needs the same Apple ID.
NextKeyboard shortcuts