I will guess that international 1st class is required to be flown to the nearest hub to the recipient, while media mail and parcel post can go surface, and the 4-lb limit is where the USPS thinks they stop breaking even on air freight charges.

I donate a lot of books to Goodwill and friends of library sales. I'm not a collector, and all the books I have kept for their entertainment value fit on a single shelf. Little by little I have been donating some of what I've bought from you to BASFA which are then auctioned at the weekly Monday meetings to help fund the group. Anything >$10 I'll probably read & keep.