I think it's the reason that Amazon sends me partial shipments even when I didn't ask for separate shipments when I order several items at once.
That might be one of the reasons, but I'm sure the usual reason is that the items are coming from different warehouses and the logistics are easier (/cheaper) for each to ship to you directly than for them to shuffle their product around so they can send everything to you in one box.
I'm not sure what the general worldwide fact is for all kinds of payment processing. But it is general practice (not federal law, as I thought before editing this) in the United States that a seller can't actually get money from a transaction paid by a credit card unless the item ordered has actually entered the shipment process.
http://www.ftc.gov/bcp/edu/pubs/consumer/credit/cre28.shtm
That's the way Amazon has always billed me, and I think it's the reason that Amazon sends me partial shipments even when I didn't ask for separate shipments when I order several items at once.