Hey @annahm - thanks for getting in touch and for your question - this was a really great one and I was able to do a bit of looking into this with our developers and my team.
You are correct that at the moment the documentation for the FulfillmentOrder resource doesn’t reflect how to create a fulfillment itself, but there is a way to do this through our GraphQL Admin API if you’d like to take a look into that method as an option. One of my colleagues shared this example application that is available publicly, that can be used as a reference for developing a GraphQL fulfillment app for Shopify if you’d like to use that if you decided to migrate to using GraphQL.
Separately, since I do understand you were looking for a way to do this through REST initially, I was also able to get in touch with the team responsible for our developer documents and can confirm they are working on adding a new example for how to accomplish creating a fulfillment through REST - I can’t guarantee a timeline specifically for this, but please keep an eye on the documentation and our release notes, which will reflect any new updates.
Thanks again - especially for your patience while we took a look into this for you. Happy to answer any other questions that might pop-up.
Best!