Where are the origins of the word shipment?

Answer 1

The English word shipment derives from Old English scip[pronounced ship], meaning a large seagoing vessel or water craft; this word later came to include the sense of sending goods by water. The -ment ending is a common way in English of creating a noun from a verb (as in payment, pavement, enjoyment).

So in the strict sense, only goods transported by water can be "shipped" and only goods sent by ship are a "shipment". In modern American usage, the history of the word is ignored and it is applied to any method of sending goods.