A Shipment represents the items that need to be shipped in order to fulfill a Sales Order. A shipment may contain one or more packages.
id - Integer - The ID of the shipment
packages - [Packages] - Array of packages on this shipment
location - Location - The shipment's location
sales_order - SalesOrder - The shipment's sales order
shipment_items - [ShipmentItems] - The items to be shipped to the customer
estimated_weight - Decimal - The estimated weight of this shipment, as calculated by summing the weights of all shipment items.