Go to Shopify Fullfilment section within API section, which its link is this: https://help.shopify.com/en/api/reference/shipping-and-fulfillment/fulfillment#update-2019-07. You can see how it has changed, in the previous API section, you can see that there were two IDs. The first ID is the orders ID and the second one is the fullfilment ID, so, how can I get this fullfilment ID and orders? If I take a full order, the response will look like this, in this section you can see there is "ordes" and "id" which is mentioned as an orders id. If you scroll below, there is a fullfilment section that is not formatted so you just have to open http://jsonviewer.stack.hu/ to have a better perspective of the text. Here you can see the fullfilments, this is an id, that is a fullfilment id. So, in this section you can see the first one is the orders id and the second one the fullfilment id.
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Shopify Tutorial: How to fulfill orders
Step 1: Go to Shopify help section
Recently the API format has changed, so go to Shipping and fulfillment/Fulfillment in the “Help” section, or just click in this link.