...The application is incomparably small.)
Unzip, run sumcalc.exe. That's it!
You can put the folder wherever you want, except system folders with permissions (e.g. Program Files), and create a shortcut if needed.
Cross-platform source and info:
https://github.com/qandak/sumcalc
Changelog:
https://github.com/qandak/sumcalc/blob/master/CHANGELOG.md