<?xml version="1.0" encoding="UTF-8"?>
<fulfillment>
    <tracking-number>1Z0000000300002879</tracking-number>
    <notify-customer type="boolean">false</notify-customer>
</fulfillment>

@payload = {"fulfillment" =>
    {"line_items" => ["order_item_id1", "order_item_id2", "order_item_id3"],
    "notify_customer" => true,
    "tracking_number" => order.tracking_number
    }}.to_json