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I would reconsider this pricing model. Flat monthly fee with unlimited calls/runs may not be pleasant for you.

OP here. I don't do any cloud computing to deliver the service. It is essentially a pure JavaScript-application running on the client.

I have a similar tool that I am working on that is basically javascript on an html page.

There are probably other ways to add value for paying users so they are less likely to just save page as:

Integrating the JS into a WordPress plugin to generate screenshots automatically for posting to social media when a post publishes.

Zapier integration to help people automate these screenshots in their workflows.

then what's stopping me from sandboxing this and .. not paying anything?

Goodwill. And the fact that people can do any number of other things to get the same effect without paying. What are you really asking?