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...
The whole thing is basically a database, simple front end, and a bit of code that generates a license key. The license is basically a signed/encoded string which my software can parse and validate. The user buys the license string, they feed it to my software as a config parameter and the software verifies the signature and validates it in various ways. Job done.
If there's any demand for something like this I'm willing to productize the whole thing.
Gumroad seemed super simple in comparison. I was concerned about it being a relatively young company and some stories about all sales being refunded because of too many suspicious transactions though (I'm guessing this is rare). Worst case you could export all your license keys + user emails and transition to another service. It's really quick to try out as well as you can make test purchases and just use "curl" calls to verify licenses.
Other ones to look at are Sellfy, FastSpring and Paddle. Paddle told me that for subscription checks I'd need to maintain my own database + server for that. Gumroad looked the easiest by a long way.
Have you considered other services? I'd be interested to know what you found. I'm with you that you don't want to be implementing this yourself. Not having to worry about bugs in your payment processing code is likely well worth the cost.
It’s not a SaaS but rather an on-premise solution, which can be an advantage depending on your infrastructure.