I have a few questions regarding (custom) invoices in combination with the API.
What endpoint are invoices (PDF?) retrieved through via the API? I digged into the API docs but could not find any invoice related documentation. Reason for this request: as a fulfillment service we need the invoice to create a corresponding packing slip.
Related second question, can a custom file be attached to a file (admin side, so no customer uploads) and then be retrieved to the API?
Mind the fact that I am not talking about DraftOrders, it's regarding orders being made by customers
Shopify API does not allow you to retrieve PDF invoices, but you can use our invoicing app Sufio to do so.
Here's a document that will explain how to download a PDF invoice for an order in your Shopify store.
Hope this helps, please let me know if you have any questions.