OK, I don't use it, but I think the catch is as follows.
You MUST purchase a CD each and every month.
You supply them with a "Want" list and they fill your monthly order off this list.
If you forget to have a CD on your list, you will still be charged even tho you did not order a CD
As far as multiple CD sets, I think the flat rate is still $5.99 per CD, not per title, so if you order a 4-CD Set you will pay $23.96
It's not a bad deal, if you want a minimum of 12 CD's per year at $71.88