And... one last update.
Since the test book worked out so well, I picked up a flat-rate box from USPS (god bless them). The largest box is $15 to ship. I packed that as full as can go with books that (a) can't be bought in eBook form and (b) probably won't be available any time soon (i.e., orphan works).
The math came out such:
$73 for 23 books at $15 shipping = $3.82 per book.
So obviously this is still not the magic bullet for converting your library cheaply and easily BUT it's a step in the right direction. I hope to have the remainder of my orphan works converted by the years' end. I'm very happy at the moment.