It was actually even smarter than that. People also forget about "media postage" rates that existed at the time. (They still exist, just fyi)

In 1999. To ship a book using media mail cost of $1.13 for the first pound and $0.45 for each pound up to 7lbs. It got cheaper per pound above that.

This meant even heavy books were relatively cheap to ship. Way cheaper than random goods.