Create an order with custom shipping using orders API

[connerludlow]

Hello,

I am trying to use the Shopify API to place an order and include custom shipping amounts in that order. I've searched around and was pointed to the "shipping_lines" attribute, but that still doesn't seem to be doing the trick. Here is the current body of my api request: 

{
        line_items: ...,
        inventory_behaviour: ... ,
        customer: ...,
        shipping_address: ...,
        shipping_lines: [
          {
            custom: true,
            price: 5.99,
            title: ...
          }
        ]
}

When I try to place this order, I am getting the response: "unprocessable entity". Is it possible to place an order with a custom shipping amount and if so, what am I doing wrong?

TIA!