Is Things a single purchase for all devices? 

– No. Things for Mac, Things for iPhone, and Things for iPad are sold separately

I have more than 1 Mac/iPhone/iPad. Do I have to buy Things multiple times? 

– No. You can install the app without additional cost on all devices of the same kind 

Are there discounts? 

– No, Apple’s App Store doesn’t offer discount options for apps which are sold as an up-front one-time purchase.