The Spree::Carton model represents how an order was shipped. For stores that
use third-party logistics or complicated warehouse workflows, the shipment
described when the order is confirmed may not be how the actual shipment is
packaged when it leaves its destination.
A customer orders two t-shirts.
Solidus creates one Spree::Shipment object, which is used to ship both
t-shirts in the order as one package.
The customer is charged for a single shipment.
Your t-shirt warehouse receives the shipping instructions for the new order. However, they realize that one of the t-shirts is currently backordered. They cannot send both t-shirts in a single package.
The warehouse ship the first of two packages, which is the first of two t-shirts.
However, you do not want to charge the customer for a second shipment. Your inventory was not accurate, and your store is paying for the additional package.
If you split the shipment into two Spree::Shipment objects, the customer would
be charged shipping for both packages.
To avoid an extra shipping charge, you can instead create additional
Spree::Carton objects for every package sent out of the warehouse. This way,
the shipment object stays unique and the order total does not change.
The warehouse ships the second of two packages.
This second package is associated with the order as another Spree::Carton
object. Although it is shipped as a separate package, the customer is not
charged for another shipment.