Ask HN: What license key service do you use for your product?

I want to generete a license key for the customers who buy my product after they purchase (e.g. with stripe or paypal). I don't want to reinvent the wheel, is there a SaaS or similar I could use for key generation and management? This is especially important because the product is an App Script add-on, so everything "administrative" that has to be built on purpose is a distraction...