It's based on dollar amount per month, i.e., $500 to $999 -- 5%, $1,000 to $1,999 -- 10%, etc., and goes to a maximum of 20%.  You'll have to decide your own levels based on your cost structure, but you get the idea.  Oh, and I don't give the discount based on a promise of volume; they have to deliver the volume before I'll give anything up.