This popular referral offer is back after being 30% off first 3 orders for a short while. Also I believe delivery is free for the first order, and $1 for the rest of first month for new customers.
Get $20 in credits when someone signs up using your referral link and places their first order over $20. Your friend also gets $45 off ($15 off each of their first 3 orders).
Edit: I just noticed some of the referral links still lead to 30% off first three orders, where as most of them say $15 off $20 for first three orders. Check the description at the top before you sign up.