Create a new fulfilment
POST /orders/ID/fulfilments

{
  "items": [
    {
      "id": "<Int>", // ID of the line item from the order
      "quantity": "<Int>",
      "warehouse": "<Int>" // required when product is track_inventory
    }
  ]
}

Edit all fulfilments
PATCH /orders/ID/fulfilments

[
  {
    "id": 1, // ID of the fulfilment you want to edit
    "items": [
      {
        "id": 11,
        "quantity": 1
      },
      {
        "id": 12,
        "quantity": 1
      }
    ]
  },
  {
    "id": 2,
    "items": [
      {
        "id": 11,
        "quantity": 2
      },
      {
        "id": 12,
        "quantity": 3
      }
    ]
  }
]

Edit a single fulfilment
PATCH /orders/ID/fulfilments/ID

{
  "items": [
    {
      "id": 11,
      "quantity": 1
    },
    {
      "id": 12,
      "quantity": 1
    }
  ],
  "warehouse": "<Int>"
}

Unmark fulfilled
Use this if you wish to delete a fulfilment and set the products back to not fulfilled.
DELETE /orders/ID/fulfilments/ID