I get their email updates, and they recently sent me out one stating that the "first Monday" codes (free rentals good only on the 1st Monday of every month) were being changed to SMS-only. So you only get one if you're signed up for text messages, and each text has a unique code that can only be used once.
Unfortunately for me, I don't do texts. I think it's dumb to pay my cell phone company 50 cents so that I can get a free rental code. That wouldn't be free.