To monetize a side project, I'd expect to need a payment platform (covered by stripe or paypal), and a license server. The latter would need a flexible configuration, in my case generate and later validate a license key for:

- up to 100 transactions for free

- up to 500 transactions for say $100

- anything exceeding that for say $500

(the numbers are random). There must be good license server freeware out there, but I can't find it. What should I google for?