My plans for the .postcard format changed significantly in ealry 2024; please visit jphastings/dotpostcard to learn about the significantly simpler tool & ecosystem I switched to building!
(My tools under that username use brew tap jphastings/tools)
brew install dotpostcard/tools/<formula>
Or brew tap dotpostcard/tools and then brew install <formula>.
brew help, man brew or check Homebrew's documentation.