A search at dictionary.com gives the following definitions for
shipment, amongst others:
- goods carried by a large vehicle
- a load of goods sent by sea
And this for the verb "
to ship"
- to put or take on board a ship or other means of transportation; to send or transport by ship, rail, truck, plane, etc.
So I think we can safely use "to ship" and "shipment" to refer to other means of transporation too!