You need to order at least 5 CDs to get the free shipping. Box sets apply toward free shipping, but are not part of the 66% off promotion. So most of the best deals are on multi-disc sets that aren't classified as box sets.
They do have some good standard prices on box sets, however, that become even better with free shipping. I ordered Dave Brubeck's 5 disc "For All Time Set" and got it shipped for just $17.99. Shop around the site. There are bargains.