Especially if you're mailing them -- you get hit hard for a heavy package, as opposed to two smaller packages. At least, that's been my experience.
Not to mention it's easier to get smaller packages to the post office...
At least this time I won't make the mistake I made coming out here: I mailed a lot of stuff other than books and took most of the books with me in the van. Media rate is wayyyyyy cheaper than other shipping methods, so I'm much better off saving space in the van for other things.