Simple Commerce is a paid Statamic Addon. It's not my full time job but I still spend a considerable amount of time working on it, so some kind of payment is only fair.
Like Statamic itself, using Simple Commerce is free during development & staging environments (refer to the docs). You only need to purchase a license when you go live.
I've recorded a quick screencast showing how you can assign a Simple Commerce liense to a site on statamic.com 🔽
TLDR of it is:
.envfile, set the license key to the value displayed on Statamic.com