Amazon offers the Zinus 12" Memory Foam King Green Tea Mattress
with free shipping
. That's the lowest price we could find by $80, although we saw it for a buck less in February. (It's also $9 under what Amazon charges for a queen.) It features 3" of memory foam, 2" of comfort foam, 3.5" of high density foam, and 3.5" of base support foam.