I need orders/invoices to have a few additional fields of metadata. (These are apparently called "metafields" in Shopify.)
These don't actually need to be user-editable. They don't need to show up during order, but I do want them to be visible in Order Printer as well as in the API (I currently use API version 2020-10).
I also need some basic logic to automatically fill these fields based on the values of other fields.
I'm getting mixed information on what needs to be done to accomplish this. E.g.:
can I do that straight in the Liquid template? It does seem to have some basic support for assigning values.
is there an existing app that doesn't just let me create fields (those do exist, clearly), but create basic rules to automatically fill them?
if I create my own private custom app, is that enough? That is, can I create that app from the Shopify point of view, create an ASP.NET Core app somewhere that connects with the API key, and then Shopify will automatically call my app to populate the fields? (Perhaps with webhooks?)