Yes, buying a bunch of sale stuff in honor of Earth Day is somewhat ridiculous. The Walgreens deal is all over the internet, but in case you haven't seen it: Thursday only, ink cartridge refills are $1.00.
These are usually on special for $10.00. We do this all the time. You don't pay if it doesn't work. I suggest bringing a few empties for that reason; there is a pretty high failure rate.
You need a coupon. It is in the weekly flyer, which should be available in your store.