I'm developing a system that receives in a single XML message an order with
all necessary details (Customer, Items, ShipToAddress).

I'm witing the Use Case for this right now and have a phylosophical question:
Since Use Cases are mainly concerned with the interaction between the actor
and the system, what level of detail about the inner workings of my app should
I include in my Use Case body?

I mean, In my case, I (my system) receive a message and I create (and validate)
a Customer object, then all the Item objects associated and lastly assign
a valude to the ShipToAddress (probably an attribute). I mean, all this events
occur behind courtains since from the actor's point of view he only submitted
one message and is expecting one response message.