How can I use Flow to automatically fulfil one product?

Thanks Paul, that’s helpful.

I haven’t yet used the Shopify REST API - in flows or otherwise - does my set up look correct?

And to get access tokens, would I set up an app as per these instructions?: https://help.shopify.com/en/manual/shopify-flow/reference/actions/send-http-request#call-the-shopify-graphql-admin-api

Thanks