I don't know why someone downvoted, 39 - I'm having quite a job finding corroboration in online dictionaries for the following.
Some verbs in English have a dual usage, where a transitive usage may be switched to an intransitive one, the direct object becoming the subject:
Bill closed the door - the door closed
Phil broke the window - the window broke
Jill cooked the ham - the ham cooked
Qwill melted the ice in a pan - the ice melted
Will sold his Monet - Will's Monet sold for over $12 million
Such a usage is called the ergative usage.
I'd distinguish this from the middle voice, a similar construction where a property of state rather than an action is being described:
Plastic windows break too easily.
Ice melts at 0 degrees C.
Battery-operated PIR alarms are selling for $12 at the hardware store.
I'm sure that the verb ship can be used ergatively, but it doesn't appear in Wikipedia's lengthy list, and the following is the only support I've found for my analysis:
ship: To go aboard a ship ... (AHDEL)
(obviously, to ship = to send by ship is a well-known transitive usage)
I'd add that the meaning of 'ship' (transitive) has been broadened to 'send by, or as if by, ship', and so that of 'ship' (intransitive) has followed, being broadened to 'be sent off'.
